We have a handy guide for this process on the GDevelop Wiki. You can even create multiple builds at the same time. You can create packages for Windows, Linux, and Mac, and upload those versions to your game's Steam page.
And that's it! You don't need to code differently for different platforms, and you don't need to pay GDevelop to publish on extra platforms, either.
So if you want to make a game only once, without writing a single line of code (though you can add JavaScript if you want), make your game with GDevelop and publish it on Steam with just one click!