Even the "false gars", AKA Hujeta gar, Needlenose, Spotted & Marble-Lined Characin, etc, all grow to around 10 inches in the home aquarium, making them far too large for your tanks.
The seven species of actual gar can all reach around two feet in captivity, so obviously that's another no.
Halfbeaks and crocodile fish may be of interest to you, but honestly, having kept both, I would say even a twenty gallon would be a bit small for their skittish nature.
Elongated, beaked/snouted fish need a lot of room as not to snap their spines or snouts from a spook into the glass. They are all also prone to jumping.
I would advise against keeping any of the above species in your mentioned tanks.
- Michael