it might not be the picking on one another. peacock bass are boisterous fish and for a number of reasons it's a bad tankmate for the gar. also it's noted that although done many times, arowanas aren't the best tankmates for gars.
1**-food. pbass eat very quickily, and chances are that the gar will be unable to get enough food
2-aggression. the pbass/aro might be punking the gar around when you're not watching. i'm not saying it's happeneing, but there's a relativly high chance
3**-gars like cool, slow moving tankmates. the quick, sudden and large movements from the bass and aro are likely to be stressing it out. more importantly, gars are very skittish fish. sudden movements are likely to startle them, sending them crashing into tank walls, and risking them breaking their backs.