Depends on a bunch of things, ie what kind of gar, wc or from the LFS, whether it was eating at the LFS etc. Mine (a Florida) ate within 5 minutes of being in my tank, but he was an adoption from a coworker so he was already pretty established.
I agree, it does depend on a lot. All three of mine ate right away, but they were about two or three inches bigger than yours when I got them. They're Floridas.
they usually eat when they are ready to eat IMO. mine sat for a day before he ate when i got him form the LFS. and he doesn't like to chase his food so that may be why...he just sits there til the fish is dumb enough to swim by and then wammo its gone..lol
it's all about waiting. at some point, your gar will eat. it took my first gar about 3 weeks before he took food. It took my second gar about 2 weeks. one thing that i've started doing recently, is using a bbq skewer and placing krill or silversides on the end of it and placing it in the tank, next to the gar. i do this because they're slow moving fish and i have them housed with fish that are faster, greedier and more aggressive when feeding. this guarantees my 2 gars a fair amount of food.