Since this has kind of become the de facto big gar thread, I'll go ahead and temporarily hijack it a bit more (sorry OP, lol). Here's a gar dissection, of sorts, sorry if it's a bit gross but it will give you an idea of how big a fish a large alligator gar can swallow. (The fish in the video is a large grass carp, not sure what the one is in the pics.) Many of the bow fishermen in Oklahoma are letting the biologists from the university have their fish for aging and diet analysis, this was one of them:
http://s749.photobucket.com/user/FullBlown77/media/VIDEO0100_01_zpsa1f55598.mp4.html