Doesn't sound like, but (probably) is. http://www.gnu.org/licenses/gpl-faq.html

"If a program released under the GPL uses plug-ins, what are the requirements for the licenses of a plug-in? (#GPLAndPlugins) It depends on how the program invokes its plug-ins. If the program uses fork and exec to invoke plug-ins, then the plug-ins are separate programs, so the license for the main program makes no requirements for them.

If the program dynamically links plug-ins, and they make function calls to each other and share data structures, we believe they form a single program, which must be treated as an extension of both the main program and the plug-ins. This means the plug-ins must be released under the GPL or a GPL-compatible free software license, and that the terms of the GPL must be followed when those plug-ins are distributed.

If the program dynamically links plug-ins, but the communication between them is limited to invoking the ‘main’ function of the plug-in with some options and waiting for it to return, that is a borderline case."

http://www.blender.org/education-help/faq/gpl-for-artists/#c...

"Can I sell plug-ins for Blender?

Yes you can, but only if you provide the plug-in and the sources to your clients under the GNU GPL license. The client then benefits from all rights the GPL offers; free to use it, or even distribute it when they wish.

Only if the plug-in doesn't work within Blender as "acting as a single program" (like using fork or pipe; by only transferring data and not using each others program code) you have the full freedom to license the plug-in as you wish."

What process the code runs in is a complete distraction, I'm going to ignore that part. It has nothing at all to do with linking or derivation.

Anyway, we're talking about plugins that upload files to steam with a simple interface, so they could easily be independent code, sharing no internal data structures.

I'm not ad-libbing, you're arbitrarily deciding to ignore the GPL, with no justification. It's not a distraction, it's the whole point.

If Blender is GPL but the plug-in interface is Python and thus only GPL-Compatible, why can't the plugin be GPL-Compatible and thus not require providing sources?

Because the critical point isn't the use of python. The critical point is that your plugin code is running in the same O/S process as the parent application.

Also I don't think that's how "GPL Compatible" works.