Sorry for a stupid question, but is it right use someone’s game files? What if I can’t contact with author of game files’ (not in GDevelop) on the forum? Maybe I should credit him in “Credits” section of my game if I use game files of someone?
It depends on what you do with it and how the author published it (under which license). More context please
I mean usual files in this forum
Always PM the author and ask him/her if you are free to use his/her code, unless the code has a license itself for instance my template has a MIT lisense so anyone can modify the code under MIT restrictions
But what if post with game’s files was created, for example, 4 years ago? And what if I can’t contact with author?
Normally the time of the code is not relevant, that is, I have code created for 11 years and it is protected by copyright, nobody can use it, but I also have code created with MIT or GPL2 license, in these cases anyone can use it and modify it.
Regarding the author, you should search for him within the forum if there is no way to contact him within the forum, you may find him on his website or on some social network and thus contact him.
My advice is that for the simple reason that if you use your code and the author is not happy with it, tomorrow you may have problems because if he asks you to remove his code from your game.
In my years of experience I had this problem with a Construct 2 code that I use for one of my games, then the finished game is published on the AppStore, Amazon Store, Windows Store and Google Play, two months after being published I received a email requesting that he withdraw from all markets, for me it was a waste of time and brutal money. So I decided never to use third party code without asking for confirmation.
But what if there’s no contract information? It will be very complicated
By default no info means default copyright. That means you cannot use it.