In BGE you can make and sell games completely for free.
You will never have to pay the BF for games you create and sell. There is nothing stopping you selling games you create with BGE, and indeed, there are games on Steam made with BGE.
However, when you distribute a game from BGE, there are also certain conditions. According to the blender foundation:
Blender and the Blender Game Engine (BGE) are licensed as GNU GPL, which means that your games (if they include Blender software) have to comply with that license as well. This only applies to the software, or the bundle if it has software in it, not to the artwork you make with Blender. All your Blender creations are your sole property.
The later specify:
In summary, the software and source-code are bound to the GNU GPL, but the blend-files (models, textures, sounds) are not.
This is in concordance with the license text:
You may convey a covered work in object code form under the terms of sections 4 and 5, provided that you also convey the machine-readable Corresponding Source under the terms of this License
This means that any code that interacts with the BGE must be distributed along with your game. Seeing as you’re writing python (a scripting language), it’s very hard to not comply. Making the code available can be in the form of a written offer rather than providing it with the game. If you have a really long EULA for your game and at the bottom put “the code can be accessed by request upon emailing {}”, I’m sure no-one would find it or ask for it…
Nothing in the GPL prevents selling the software. And indeed there are companies that sell blender. Literally they download blender from blender.org, and set up a website where you can pay to download it. It’s not against the GPL, they can do it (example: 3dmagix. Go look at those interface screenshots. In this case, they violate many other copyright licenses on their website, but not the license of blender itself. You can see what the BF had to say about this here.).
However, for a game you make, other people cannot distribute your game without your permission, because the models and textures are not GPL’d - so any distribution of your game without your permission is against the copyright of those items.
Interestingly, the GPL license text sates:
The “Corresponding Source” for a work in object code form means all the source code needed to generate, install, and (for an executable work) run the object code and to modify the work, including scripts to control those activities. However, it does not include the work’s System Libraries, or general-purpose tools or generally available free programs which are used unmodified in performing those activities but which are not part of the work.
I wonder if BGE falls under a “general purpose tool” and “generally available free program”?
The assets you use
Yes, you have to pay attention to any assets you use when creating your game. For this reason I only use items under CC0, or GPL. If I can’t find items with this license, I make them or pay someone to make them for me.
As a rule of thumb, never download or use anything unless you know the license of it.