1999 GMC seirra misfire

Provided that your tune up went perfect, (and that the vehicle acted the same way before your effort) you need to locate the cylinder that has the "Misfire." Please consider that the "Misfire," could in fact be a faulty/clogged injector, as these will commonly seem like an ignition issue. Once again you need to locate that particular cylinder...then pull the plug on that one...inspect for lean or rich condition. You should be able to determine where the problem lies. Now, don't forget that you may have a cylinder which is low on compression (that's why the truck feels ok when you get to higher revs...it carries that cylinder with the momentum of the others). Could be a bent/burnt valve...and I hope not...but something to consider.

Check compression this issue sounds like a piston ring issue or even a head gasket issue. Check compression when engine is hot and cold and compare readings. If it checks out ok then it could be an intake issue. If they change I would put a head gasket on it.