I have a Silver Aro with Gars living peacefully for about 6 months..
I did some research before going for this combination and the general consensus is that the Gars will try to eat any silver fish if they can.
In my case, it's working because the Silver Aro has always been nearly twice of the size of the Gars and hence too big for the Gars to attack him. (And my specific Silver Aro is very very docile.)
(At the beginning (6 months), the Silver Aro was about 8"-10" and the Florida Gars were about 4". Now, the Silver Aro is closing on 18" and the FL Gars are over 12".)
In your case, the Gar is 6 inches and the Silver Aro is 4.. So, as the Gar is larger and will grow faster than the Aro, this combination will most probably not work.
In my opinion, with a larger (~10") Silver Aro, this combination will have much better chances of working in the long-term.
Good luck,
Sid
PS: Besides compatibility, there is the issue of tank-size. Absolute minimum 6'x2.5' footprint (220 Gal).. Ideal: 8'x3' footprint.