I already got prey on steam, you can buy mooncrash as a standalone on steam, but i can't do that on GOG, you have to buy both. Why ? Please allow me to buy mooncrash.
Same for ost of some games like shadow tactics.

I don't think you can buy Mooncrash as standalone on steam.
Just buy the digital delux version on GOG while it's still cheaper than Mooncrash itself on steam.