The best use I found for Gar is a strange team under IG88 lead.
IG88, Gar, Range Trooper, Storm Trooper, Shore Trooper.
Range stomps, and the 3 troopers get counter, and Gar assists on each counter-attack. Mod him for Crit chance. He will stack dots like crazy and the other team just melts from all of the dots. Ive seen up to 8. I posted it before in a thread called "Team Bleed" or something like that if you want to see the screens.
You can replace anyone on that team with other characters that counter. ISC, Gamorrean Guard, Sun Fac, Zaalbar, Kit Fisto, L3, Vandor, etc. Shore works best because he can heal, but people use him on other teams usually.
Bottom line Gar is FUN to use, but he doesnt really fit very well in other places. I wish he would get a trooper tag. I love to theorycraft - but Gar is a challenge.
Edit:
https://forums.galaxy-of-heroes.starwars.ea.com/discussion/173592/team-bleed