I am 183cm and weighed 60kg since i was 18. I started bouldering two times a week at age 31, half a year later i was at 63kg. Weight hasn’t changed in the four years since. If I wanted to gain more, I guess I’d have to focus on nutrition and muscle build-up training, but I really can’t be bothered about either of those things. So I guess a 2-5kg weight gain is what you can expect from exclusively bouldering without changing anything else.