assassin snails sound like one of your best options, but i have never tried them. i would avoid any of the chemicals as they are bound to have some reaction from the gars (primitive fishes in general do not react well to several meds that may work with other more derived fishes).
i've had snail infestations in the past, and really the issue is primarily aesthetic; the snails help break down uneaten food and actually help maintain the biofilter balance in the tank. also, you can put in as much lettuce, etc that you want, but that will never eliminate the problem from your tank...for every 50 you may see above gravel, etc...there are usually 100 more hiding.
puffers are not a good idea as they will either harass the gar, or if the gar is big enough, it will just eat the puffer.
good luck with what you choose to do, but i'd stay away from chemicals--
--solomon