making a Steam account to give as a gift

You can absolutely no problem have an account for him, but he needs steam to be able to run the game.

Well, he wouldn't be able to use the steam marketplace without funds anyway. I feel like he would be able to access it anyway if he wanted by just making a new account.

• if that would bother you then why not discuss about that with him? 🤔

You could family share the game from your account to his, but you would be forced to quit the games you play when he plays and the other way around.

GOG copy can work. Its drm free so there is no need to run client and currently its on sale with 75% off.

It's perfectly fine. You won't get in trouble for making another account or anything. I have a child cousin and I created an extra account just so he could family share some lego games from me during lockdown, under supervision from my uncle.

I've read that you're already using family share feature in your account so:

You can create 2 steam accounts

First account, buy the hp games
Second account, connect it to family share so your nephew won't be able to access marketplace/store in general (and you, setting up the family share can control what they can access. For example, library only

So you can build up your nephew's library in the first account while using the second account. If your he/she is in the "right" age already you can give him the first account with complete access to everything