Whenever a certain other Titan is around, Gar's brain does not seem to function normally. Look at how he was acting at the beginning in his "day in the life" issue of Titans. He went to the other side of the US, far away from this other Titan, he acted almost normally. In fact, he didn't even seem to think about or mention that Titan. Now that person is around again and he's acting strange again.
Also remember what that certain Titan did to Wally West's emotions back in New Teen Titans. Yeah, it doesn't have to be a conscious effort on that person's part... but that doesn't mean that person's undetermined powers aren't effecting Gar's emotions and personality whenever they are around.