Would it be bad to steal some money from my brother's friend's wallet because he was hogging my gta 5 that I just got?
Of course it would! Won't he notice the money is gone?
His wallet is so fat he won't notice.
How are you going to feel about yourself after stealing?  If you don't like it when someone else steals from you, then may don't make other people feel that way.
How am I going to feel ? I'm not even gonna speak to him again and I'm going to use the money to buy a new gta 5.
It sounds like you are trying to find reasons to justify stealing from him no matter what I say. But, it is wrong to steal and it's even worse that you are taking advantage of a friend. Just ask for your game back or tell him he has to replace it.