The compatibility would not be part of the license in any case. Some licenses could be considered compatible with GPL (specifically the CC BY SA). However, CC generally suggests that the licenses are used for non-software creations.
I think the problem is that both GPL and CC-BY-SA require you to license derivative works with that same license. And sometimes it's not easy to tell if something is or isn't software in a clear way.
For example, if you modify an image of a map licensed with CC-BY-SA for it to be used in a level of a game licensed with GPL, which license should you use for the derivative work? Is it data, code or both at the same time? What's the status of an image or a sound made with a script (think about fractals, for example)?
Both licenses are "viral" and require you to use that and not the other. You are on a deadlock.
This is the general problem with share alike clauses and GPL is known for adopting a very limited interpretation of their Share Alike clause. They have recognized CC0 as being compatible with GPL, which is a step. In your example, I'd say that most definitely software cannot be under CC. My perception is that the software license always prevails. Now the image in the game could be implemented with an authorization from the author (it's the best way to avoid problems in any case).
I'm in complete agreement with you, but I think we wouldn't need to rely on personal interpretations and recommended practices if the SA licenses stated clearly something akin to:
If a copy or a derivative work of the licensed material is to be included in a piece of software under the General Public License or other strong copyleft license, then it must be licensed under that license for compliance.
I don't speak legalese, so I'm sure that text is full of holes and possible misinterpretations, but surely a group of experts could come up with something legally viable in that spirit.
CC-BY-SA-4.0 allows for one-way compatibility, compatible licenses list maintained outside license (currently none, but hopefully GPLv3+ in the near future; bilateral compatibility with the Free Art License is a possibility also). 3.0 only allowed for bilateral compatibility (currently none, and none expected).
