you have been lucky , gachua are one of the ones that people seam to be able to get away with the most and tolerate the widest temp ranges , dont grow to large ,will breed fairly easy in the aquarium and an excelant first snakehead.
now the only thing wrong is the gar , depending on what type you may be ok , in genral i would not expect a gachua to attack a gar unless a pair forms , if a pair forms then he may well be in danger, the real probem is the potential size of your gar ,again depends on species ,but it may grow large enough to eat the gachua. but you could always move him on if a problem occurs.
as you are probably unsure of where the gachuas where caght i would set up at around 24-25 deg.
gachua do like to hide and feel secure ,they are very jumpy if you do not give them plenty of hiding places ,and of course make sure they can not get out .
good luck with them ,iam sure you will love them . they are a very good snakehead. and far better choice than ornatipinis
cheers col