ive caught gar and flats in the same area on the same trips never really noticed a pattern,i could catch a gar and 5min later land a flat.

yeah its not really scientific

in one of my most recent trips we landed carp  3 gar and 19 flats and 5 drum and then some really nice channels all mixed in together

They both are predators and you will find them in the same places hunting for the same food.  I think they will haunt more oxygen depleted water than catfish but not always.  They are extremely hard to hook with that hard beak of a mouth but I have landed a few.

On nights that I knew gar were stripping my bait, it was hard to catfish since the bait hardly had time to hit the bottom.  I am sure the catfish are there too, just not as fast as a gar to the bait.

Burnsville Darrell told me once that they are even good to eat although hard to clean.  I cannot touch the things without them cutting me to pieces so I will admire them from a distance since they are an interesting fish to look at.