After entering the second "portal" the game just coloses and opens a windows prompt telling me I need "another application to run this steam", and now everytime I open the game, the same happens after the company names appear, does anybody know how to fix this?

Solved by GoG team:
If anybody else has this problem, the solution is going to your library and downloading the 1.0.2 version installer and installing it this way, since the GoG Galaxy 1.0.3 version gives this error. They are trying to fix it, but for now this is the solution.