Karcher K2.900M spray gun leaks. Not sure which replacement gun to get or if there is anyway to fix it. Thanks

This is normal. The high-pressure car washer has certain dangers. If the water column of his spray gun is aimed at people, it will stab people. If the switch is turned on and spray water, then the pressure of the spray gun head will be sprayed around, which will accidentally hurt people. people, so it's designed this way.

Hot water, not boiling, pour over the joint and then try. I take it the joint is the standard push in type. When re-fitting, a smear of grease around the joint would not do any harm.

use teflon tape

Leakage from the machine itself, can be from faulty O ring Gaskets.
If the leaks are from the gun, one can service the gun. Please note that a gun leakage will cause the machine to start and stop many times. These are a few points that I can help you with. If you described from where the leakage is coming, I could be more helpful.
Regards,
MG.

the gun needs replacing, non servicable, interchangable with many brands

It depends on were the water is leaking from on the gun. If the water is leaking from were the high pressure hose goes into the gun it could be that on oring as been damaged. If the water is coming from the trigger you may need a new trigger gun.

If you tell me your model number (1.xxx-xxx.0) i can tell you the part number that you will need to give to your Karcher dealer/supplier to replace it.

Not too much pressure, a bad gun.
Unfortunately you will have to replace the gun. About $65.00 Canadian

The o-ring and the plastic insert is broken. They may not have a part replacement, may have to get a new gun. Take the gun apart and you'll see the problem.

These units have pressure valves that automatically turn off the pump once the pressure is high enough. The pressure switches do fail and sometimes stick and act as you've described.

Your posting is incomplete, so I'm not quite sure what we're dealing with here.  I understand the machine and the high-pressure hose are Karcher - right?  Is the gun also a Karcher part?  If all are Karcher parts, they should fit unless the screw threads have been damaged.  If they are not all Karcher parts, they will not fit because Karcher connectors are different from almost all others.
Please post again if this doesn't help, giving as much detail as possible, and I'll see what else I can offer.