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.
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.
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.