I'm training regularly and I'm already much stronger but I still weight about 64kg (I'm 1,83 meters)
You could take some of my weight lol jk. I wish had you problem. My suggestion is if you want to gain some weight but keep the muscle you've built is try eating junk food ( or just eat like we southeners do lol)but keep doing your workouts to keep your muscle. Or you could do what my ex does and drink that Muscle Milk stuff. It helps build muscle but it also puts on weight from the extra muscle. That's my problem is I like to many sweets and I love some good country cooking lol. If it doesn't work for you so I wasted time but hopefully one of those will work for you.
Read "want to gain or lose weght:read this"
It's in the topics, I just bumped it to the top of recent topics.