One benefit of intermittent fasting is having more freedom in the foods you eat and being able to sit down to larger, more satisfying meals without feeling like you have to strictly count calories. But IF doesn't give you the green light to eat as much as you want. If you eat in a calorie surplus, you will gain weight.
Overeating during your eating window is common for beginners, and this can happen if you jump right into fasting or if you choose a fasting window that's too long. It's OK to take several weeks to build up to fasting for a certain period of time. If done correctly, you'll hardly feel hungry during your fasting window, so once you hit the time when your feeding window starts, you won't feel so famished that you want to inhale the entire contents of your kitchen.