I like Gary in Master League. My only real weaknesses are against Groudon and Legacy Dragonite. WF/HP/C is the one I have and he also does fairly well against Lugia and Mewtwo with the Crunch. Have also lost to Heatran before and Gary helps against it but does get hammered by Stone Edge.
My team consists of 3 of these mons.
Ttar B/C
Machamp C/DP
Dragonite DT/O
Raikou TS/WC
Metagross BP/MM
FYI, no legendaries other than the weekly ones.