I am in the middle of my AEM Workhorse install. When I removed the stock intake I noticed the air restriction gauge was gone... I guess it fell out?? Can I just plug up the hole and drive on or do I need to get another gauge to put in the new CAI?

You don't need it. Replace it if you like.
But at least plug that hole ASAP. Unfiltered air is being sucked into that hole whenever engine is running.

What do I plug it with??? I cant find anything that fits. I'm about to go to Home Depot and look.