Obvious things to check: are any coil packs unplugged? Were any sensors unplugged? Any hoses removed and not replaced? - including the few vacuum lines there are?
More troubleshooting:
Clean the throttlebody if not recently done.
To troubleshoot the misfire specifically: remove air tube to get access to coil packs. Start vehicle and unplug one coil pack at a time. If there is no change in the idle - that's a suspect cylinder. If the idle gets worse, that's a normally functioning cylinder, plug it back in. Once you've identified the suspect cylinder(s), swap the coils (but not the plugs) to known good cylinders and start over. If the misfire follows the coil, it's the coil. If the misfire stays with the cylinder it's either the spark plug or the wiring to the coil.
Also, when was the last spark plug change (at most 100k miles)? Are the plugs correct, ACDelco 41-103, and verified genuine? Genuine 41-103 spark plugs do not need gapped, they are gapped from the factory. Also, clean the spark guide (the spring between the coil pack and the spark plug) if there's any "schmutz" on it. If you notice any fluid around a spark plug, soak it up with a towel (paper or otherwise) before doing anything else. Apply dielectric grease or petroleum jelly to the coil pack gasket before reinstalling to help prevent water from entering the plug well. It is not uncommon - but not desired, either - for there to be oil in a plug well. If it's excessive, you may want to replace all of the gaskets in the valve cover - 1 outer perimeter gasket and 6 plug well gaskets.