If I had an auto clutch I'd expect it to idle.
Pretty much all OEM engines have excessive squish clearances; it's not Gas Gas specific.
I don't think you have a vacuum leak. Vacuum signal was in relation to the intake charge/velocity, impacting on the carbs ability to pick up fuel from the jets at low revs.
Make sure you always work with the engine at operating temp, a clear oiled air filter, fresh fuel, and a spark plug that is gapped correctly and not caked in carbon.
Get the bike to idle nicely in neutral as your first step. You want to be able to apply some throttle and have the revs drop back down and find the steady idle. Going leaner it'll want to hang and race for a bit. Too rich it'll flame out and stall.
Once you have it doing that nicely with the idle screw nice and low you can start bumping the idle up a bit to find the idle you require in gear, still paying attention to make sure your revs drop nicely.
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