I would keep an eye on the gar-pike interactions. If it really seems like the pike is bullying the gar, it may be better to return the gar, but based on my own experience, they should be ok, as long as you make sure the gar gets enough to eat. In my tank, bullying wasn't a problem, it was the gar learning they had to be more aggressive feeding. Now they're such aggressive feeders, that most times the pike stays out of THEIR way.
Just keep an eye on them, and be ready to remove one or the other if there's obvious aggression issues.