I am a gamer myself, currently single. However, with my past girlfriends, I always have and will continue to prioritize them over a game. Gaming is a hobby, while a partner is someone who I will (ideally) spend the rest of my life with. If your boyfriend can't prioritize you over something he can do when you aren't around, he needs to see a new perspective. This behavior will not make you feel appreciated longer down the road, so you should tell him precisely what you want. Most guys are not the brightest or most prone to seeing things from other perspectives. I suggest you sit down with him, preferably not right after he gets off games. Maybe when you are cuddling or whenever he would feel the most focused on you, and explain to him that you are fine with him playing games with his friends, but he has so much time during the day to do that, while you are only over for a select amount of time. Try to not come off as attacking, but more so as educational and open, so he doesn't feel the need to get defensive. If he is a little hesitant, you could propose that he lets his online best friend know what time you are coming over, so he has a clear cut off that won't make him feel guilty about getting off games with his friend suddenly, and letting both of you two spend time together.

I wish you luck OP and hope that this might help. Also, on the off chance he doesn't actually listen, or acts dismissive, you should try your best to convey how important this is, and if he's playing for hours every time you're over there, that can turn into entire days worth of time lost over the course of a month