Depends on the species of Gars and the species of Aros..
As a general rule, Jardinis can not be kept (long term) with any tank-mates..
As a general rule, Alligator Gar can not be kept (long term) with any skittish tank-mates or tank-mates which can be eaten.
There are skittish and delicate gars (like long-nose gar) which has a tendency to freak-out and hit the side of the tank when kept with aggressive fast moving fish.
All in all, there are a lot of combinations, some have higher chances of working than others.
Personally, I have kept Silver Aro with Florida Gar and Alligator Gar. (Alligator gars are not with me any more). This worked for me because the Silver Aro was docile and didn't spook the gars. And the gars were always about half the size of the Silver Aro and hence didn't threaten the Aro. I still have the Aro and the FL gars together in a 200G.
As Brian said, please check with the experts in the Gar section.
Good luck,