What do I have to do to drain and disconnect GG40T6A model

You will have to take the case off and look for where the leak is . Then replace that part .

http://waterheatertimer.org/Water-heater-will-not-drain.html
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Turn power off, turn water off, open tub spout, Connect hose and open drain valve?
Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Adjust temperature?
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html

Troubleshoot by type of gas control?
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Maintain water heater:
http://waterheatertimer.org/How-to-maintain-water-heater.html

Add a comment with more information as needed

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Hot water pressure relief valves serve two functions. The first is to remove high-pressure water from the hot water tank. The second function is to act as a high temperature valve to remove water that exceeds a predetermined point for the valve. Both of these relief settings are preset at the valve factory. Many pressure relief valves will come with a metal tag that identifies these release points. If you're attempting to remove the hot water heater pressure relief valve, pay careful attention to ensure that you replace with the correct valve.

Your hot water heater has a pressure relief valve safety mechanism. When pressure inside the water tank exceeds certain limitations, the valve opens up to allow the pressure to be released. A faulty pressure relief valve will not release excess pressure, which could cause extensive damage to your home. You should test the hot water heater pressure relief valve once every six months to ensure proper operation. If the pressure relief valve fails the test, replace the valve with a new one immediately

Locate the pressure relief valve on your hot water heater. The pressure relief valve has a lever handle on top with a drain pipe attached to it. The pressure relief valve is most often on the side of the water tank toward the top. Newer models of hot water heaters have the pressure relief valve on top of the water tank.

Inspect the area around the pressure relief valve for signs of leaking water. If there is leaking water near the relief valve connection, replace the relief valve. If no leaking is evident, proceed with testing the pressure valve.

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Examine the area below the pressure relief valve drain pipe. Most water heater closets either have a drain in the floor below the pipe or the pipe is inserted into another pipe to direct the water out of the home. If the end of your drain pipe is not in another pipe or there is no drain in the floor, place a bucket under the drain pipe to catch water when you test the pressure relief valve.

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Pull the lever handle on the top of the pressure relief valve up with your fingers. A working lever will lift up without difficulty.You should begin to hear water going through the drain pipe. Slowly release the lever to shut off the pressure relief valve. The pressure relief valve is good. If you did not hear water running through the drain pipe, lower the lever on the pressure relief valve and replace the valve. If the lever is difficult to raise, do not force the lever. Replace the pressure relief valve.

Now with all that said if you have a closed loop water supply system and you have a pressure vessal/expantion tank it may have gone bad and is making the TP valve leak. you can call your water provider to see if you have closed loop system.

First shut off power then remove acess cover and carefully move insulation out of the way so you can see around elements.Check to see if leak is coming from around element.If it is determine what type of element you have. Either screw in or bolted in. Both have thick rubber gaskets that can be replaced by draing water heater and removing element then install new gasket and reinstall element.
To drain heater shut off valve on incoming cold water supply line ,make sure electric power is off. Connect drain hose to drain valve and put other end to floor drain or sump basin.You can open faucet on hot side near by to prevent vacumn and drain. If you have screw in type disconnect wires,marking them for reattachment,then use large open end wrench or pipe wrench on hex section of element and turn to left to remove. If you have bolt in element spray bolts with wd-40 or similar and carefully loosen bolts then disconnect wires and remove element.install new gasket and reverse to reinstall. I would recommend that you replace elements at this time if you are sure this was the source of the leak.
If you don't think it's leaking around elements then check fitting on both water lines to water heater to be sure they are not leaking and seeping down thru insulation and leaking out of the bottom of heater. Next check around drain valve for leak and if it's leaking you can try tightening it a little or replace it with 3/4 x2 brass nipple and 3/4 inch female iron pipe boiler drain. Use teflon tape or pipe joint compound on all threaded connections.
After you have eliminated all other sources for leak then it is probably the tank itself. If it is less then six years old the tank should still be covered under warrenty. Most warrenties will give you a new heater but installation will not be covered unless you have extended service coverage.
I hope this helps you some. Thank you

12 year old water heater is getting near the end.
Some tanks last 20 years, but that's the exception.
To find exact age of your brand water heater:
http://waterheatertimer.org/pdf/WaterHeaterSerialNumberDecoder.pdf

Leaking TP valve:
1) Lift TP lever and gently lower back into position to see if that stops leak.
Occasionally something gets stuck in the valve.
2) Sometimes water heater temperature gets turned up because heater is not producing as much hot water as before, and higher temps cause TP to release pressure because of overheating.
Dial temperature down to 120 degrees.
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html

3) If steps above don't fix problem, replace TP valve >> TP valves have 125 or 150 degrees rating >>> older tank shouldn't be running above 125 because higher temps cause more pressure on tank, so thermostat should be lower than 125 degrees TP rating.
Buy replacement TP at any hardware store + a roll of teflon tape to seal pipe threads.
Turn gas control to pilot ... this will keep pilot lit but burner will not fire.
Let water cool, and then drain some water out of tank.
If TP valve is on side of tank, then open drain on side of tank and drain water until it is lower than valve level. Open bathtub spigot to let air into line.
http://waterheatertimer.org/Water-heater-will-not-drain.html
Be careful around water that is higher than 120 degrees F >> you can get burned quickly >> apply cold water to any burn.
Use pipe wrench and unscrew TP valve counterclock-wise.
Install new TP and put teflon tape on threads.
Re-fill tank completely and open tub valve on hot side until water is running >> then turn gas control back to original position.
Do not fire up any water heater before tank is completely full of water or this could damage tank.

To read more preventative information about water heater leaks and age problems:
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html

The tank has to be full of water before turning on the circuit breaker.
If the elements are not submerged in water, they instantly burn out.
http://waterheatertimer.org/How-it-works.html
http://waterheatertimer.org/How-to-test-water-heater-element.html

Check the breaker and top thermostat and make sure they are not tripped. Move onto replacing elements if above is not the problem.
Turn water and power off to heater and attach a water hose to bottom of heater to drain down tank. May have to open hot side on a facuet to get it to drain.
Once drained remove elements with 1 1/2" found at local hardware stores. once elements removed and new ones in place. Turn water on and get the air out of the system. Once water is flowing from the hot side from all of facuet's in house.
Once all air is out now yaou are resady to turn power onto heater.

Look and see if your pilots flame is touching the themocoupling ,if it isnt then cleane your pilot orfice.If that dosnt work replace thethermocoupler.I would also check the draw. Hold alit match or a cigerett up to the draft diverter (the air space between your heater and the vent. If it dosent draw the smoke up the vent it meanes your vent is stoped up or you need to raise it.

look under camper at floor