My check engine light came on then I got a message gascap

Start by rebooting the as explained regkyl.
If issue returns then get a new gascap and reboot again. The seal on your cap is maybe worn out.
If issue still return then you'll have no choice to go and have the sensor checked and hope the problem is not with the cluster...

Try tightening the gascap. Go to an Autozone, Advance, or Oreilys parts store. They will give a free scan and it probably will show that there is an Evaporative emissions system leak. A leaky gascap can generate this Code.

basically what you need to do is to check the oil gascap see if it has any type of dirt or anything under, if so just clean it off. the gascap should be on top of engine where it says OIL. or the gas cap could be rusted and may have to be replaced

The gascap light means you may not have the gas cap on tight, or it may be faulty. The check engine light means the computer has found a problem that will affect performance and-or emissions.
Such as a leaking gas cap.
If the gas cap is ok, you may need to get the computer scanned for trouble codes to proceed with testing.

The Code 455 is showing the large leak in the Evaporative system (gascap) and the Code 1111 shows there are no defects.

You can get these codes wiped off with a scanner free at Autozone, Oreilys, or Advance. They may clear by themselves after 35 startups.

It would just appear to be a latent record of the old condition before and after you tightened the gascap.

There are a couple of different issues on this.

First off, the actual cel (check engine light) code would be extremely helpful it will be Pxxxx

Other than that, a general troubleshooting guide would be to check your gascap.
Remove the gascap, check for cracks in the rubber seal
Check for dents etc on the gas filler neck which would prevent a good seal
Reinstall the gascap ensuring it clicks a minimum of 3 clicks
If this is your problem, your code should go away in a few days. If you find problems with the gascap or filler neck, etc. then go ahead and repair them. This is the most common cause of evap issues.

The other issue i have seen with the evap system on a '01 blazer is the pressure sensor on the fuel pump going bad. Good news is that this part is roughly $40 dollars. Bad news is you may have to drop the fuel tank to get to it since it's mounted on the fuel pump most of the time, and the fuel pump is on the top middle(ish) of the fuel tank. I don't recommend dropping the fuel tank full. It'll weigh a lot. (28 gal x 6.1 lbs) + tank ~75 lbs.

the check engine light is most likely evap system large leak which is what causes the gas cap light it probably needs a nvld pump or it has a leaking evap tube under th vehicle.

Hello, what did someone do to repair it? Here is what you need to do. Go to Autozone or Oreillys and get a free scan. If it still shows gascap then ask them to clear the onboard computer. With the engine off, tighten your gascap again.

Everytime you put on the gascap loosely, you will get this alarm and it will stay on 25-35 Startups or until you get the computer cleared.

However, if you eliminate the gascap there are other problems which you may have that require a special tool called a smoke machine. You have a leak in one of your pollution hoses for the fuel tank. Only the smoke machine can find a small leak like this. Once you find the leak it is a simple matter to fix.

You can call around to find a repair shop with the proper equipment. I hope you find my solution informative and feel well served.

You may have a faulty gas cap. If you are tightening the cap down securely the light will go off after the car is driven about 40 miles. I suggest installing a new gas cap which should cost about $15 at your local auto parts store. *** Please rate me on this answer.***

It might take 2 or 3 gas fill-ups before the light goes off. Don't worry.