well, i played the games and saw the manga and i noticed how much he hates weak pokemon and trainers and would stop at nothing to being the best. that and he's actually Giovanni's kid (MAJOR DRAMA BOMB).
he did steal the pokémon and ran into Lyra even. he's getting his jerk moments don't worry xD but yeah even in my firered I'm not puting Gary as a total jerk so yeah. poor kids are just misunderstood. lol