How can i gain weight for basketball?

I need to find a way to gain weight. I play everyday at the park near my house and usually i play one on one with some people. it wasnt until saturday where i lost badly. (15-21) im 16 and 5'9 and only 120 pounds. the guy was like 2x bigger than me and he just kept backing up near the rim and shooting fades and i couldnt block him. i eat 130 grams of protein everyday and eat 4500 calories along with working out at the gym 4 days a week. each time i gan pounds though, i end up losing it whenever i go on vacation or something. for example i gained 8 pounds in 2 months and lost it all in 4 days when i went to LA. so how can i gain weight for bball?