Depends on the species of gar... Longnose, shortnose, spotted and Florida gar do not reach sizes any larger than largemouth and with their mouth size don't get close to feeding on the size fish largemouth can. Now alligator gars are another thing all together. They are not very common in most places though, as some of the other species are...

Flatheads growth to over 50 lbs regularly and over 100 lbs. Hardly close to comparison to a 20 lb bass. Obviously they can eat you out of house and home in a pond. Northern pike can cause issues in smaller ponds as well. One should be very careful stocking northern pike in a pond.

Perch traps can be efficient in removing a lot of stunted fish in a hurry... Let us know how your progressing....