I have responsibility over my 5 year old brother quite often, enough so that he calls me mom, and I deal with this almost daily and it's extremely frustrating and I don't know what to do, when I ask my parents they either want me to scream at him or beat him. And I disagree with that type of discipline, so I'm asking here. He hits me too hard when playing, or is pretending to be a dog and bites me somewhere and it hurts, or he will be somehow causing physical pain or uncomfortability to me and when I tell him "Please stop, it hurts gabe" he laughs and keeps doing it, despite how serious I make my tone, whether or not I'm looking into his eyes, or if I'm even trying to show through my face I'm in pain. He thinks it's a game and finds it funny. He will do this to my younger sister too, nothing really makes him stop. Now he is behind when it comes to verbal communication, he cannot really communicate with everyone back and forth in full sentences. I feel like this is important to note when addressing this issue since I cannot really have a deep conversation with him about how when someone says "ouch" or "stop" he should stop doing that to them because it hurts him and that's not nice or funny.
Has Gabe been evaluated for his language delay? That can certainly play a part in him not understanding to stop. I have a kid with a language delay who had similar issues. I would put on an exaggerated sad face and say "Ouch!" (The less words, the better) Now, if he is autistic, he might not be able to read your facial expression. So keep that in mind when you are doing this. Also keep in mind that it might take a really long time for him to understand.
Along with the ouch, I would also stop all play immediately. Nothing happens after the hitting or biting. I started with 2 words: "no hit". I think even 5 words are too many for some kids with language delays. It sounds like gibberish to them. We started with single words and built up to "please stop, that hurts [kid name]" over the period of, well, years. Be consistent with your boundaries and conveying that he hurts you, and expect it to be a process.
Now the most important part. If he hasn't been evaluated for a language delay and neurodiversity (autism or ADHD or similar), he really needs to be. The fact that you say your parents will just beat him for hitting is worrying. Will they initiate an evaluation? Generally, evaluations can be initiated through his school or his pediatrician but that really depends on where you are. With my kid, as his language improved thanks to intensive speech therapy (and other therapies), we were able to use other discipline techniques such as timeouts.