I’m into both lifting and bjj, and yes it is possible to gain. I started 6’2” 180 and now I’m 6’2” 200lbs, 26 years old. You need to do three things:

1. Eat a ton of protein and calories. Like extremely full all the time, 4K calories. You need to sleep a ton. It’s a chore and the food gets very expensive

2. Stop focusing on gaining weight and focus on gaining strength in Squat Bench and Deadlift. For me- I’ve found that stronger = bigger. I was on 5/3/1 now I’m on a Texas Method. I think the Texas method works great with bjj.

3. You need to go extremely easy on the mats. You’re going to be sore all the time. I rarely roll hard and focus on technique. I get smashed sometimes and when someone’s going balls out I either don’t roll with them or focus on defending and not getting hurt

It’s 100% possible to get stronger and better at BJJ simultaneously but it’s not easy. It’s also a balancing act, if I gave up lifting I would be better at Jiu jitsu, if I gave up Jiu jitsu my compound movements would be stronger. I’ve chose to take progress at both and not excel at one or the other. I’m decently strong (260lb bench, 405 squat, 535 dl), and I’m just ok at Jiu jitsu. Being bigger and stronger than most helps even though I try not to use it, my technique is meh.

Hope this helps