You won't gain weight if you go to the gym after the New Year's feast! Because those calories you have gained will need to be burned, otherwise fat will begin to be deposited in the abdomen, sides, less often in the thighs, etc. It also depends on your structure and metabolism. You can drink L-carnitine, it helps burn fat during exercise, you need to drink it 30 minutes before exercise (After it you really want to eat, so close the refrigerator with the barn lock !).
Of course not, you can quickly gain calories, but it will pass. There will be no noticeable changes if you overeat a couple of times, except for heartburn, bloating, etc.
Weight will not be gained