I would recomend you do a lot more research before you buy any fish. Yes a 20 gal may be fine for a few weeks or even months but what is this "bigger tank" he will be moving into? Your spotted gar will grow to 3ft+. Also due to the rigidness of their scales and body they have a harder time turning around than most fish.
I don't think it's very good practice to feed your fish food that your mom might be cooking up. But that is just me. I didn't mean for this to be a bash you post its just a lot of times people are not informed and even if you think your 125 gallon is a big tank in the monsterfishkeeping world it really isn't