Gaining weight even when calorie controlled due to exercise is possible, but you will lose fat percentage while gaining muscle mass thus increasing base metabolic rate. Doubly good.

Which is why you should get one of the approximate fat percentage meters. (Clamp better than electric but less convenient.) And maybe run a DEXA or volumetric body composition scan sometimes.

Another good easy measure is the belt size, for obese people. Smaller is better. Or waist/hip ratio. Requires a tape measure, that's it.

If you exercise and weight stays the same but belt size goes down, you're on the right track.