Here it comes, the dreaded "it depends on the fish" answer
I'm 99% sure geos would work with my gar (other than brasiliensis which are very aggressive). Depending on what species you get you might be ok, but a lot of geo species have coloration that will attract your gar's attention (talking about silver spangling etc). Also keep in mind that your geos will grow slower than your gar; grow them out large or buy them large.