So 24fps is what I'd do, especially if the 5D footage takes up the majority and you're okay with your GoPro footage being slowed down a bit.
This brings up another concern, though. Hopefully, rather than just "being okay with" the GoPro footage rendering as slow-motion, that was the intent from the beginning.
Frame rate is something that has to be thought through for the entire production/post-production process. Choose your time base according to the aesthetic you want, in this case 24p to get the "film" look. That should be the frame rate that is used in the majority of shooting, and is the frame rate that should be used in editing and output*. If a "stunt" camera like the GoPro is used, 24p should still be the standard.
If, however, the stunt camera is being used specifically to get slow-mo, then a higher frame rate will be used on that camera. That footage will then be slowed down in post to conform to the editing time base. The number of frames captured will not change, but fewer of them per second will be played. That makes for smooth slow-mo.
If shooting for broadcast and wanting the standard "video" look of television, shoot and edit in 60i (or 60p), and if shooting for slow-mo set the GoPro to 120fps.
The OP is in France, so I assume SECAM should be considered which, like PAL, runs at 25p/50i. For online or DVD/Blu-Ray distribution, 24P is fine. For broadcast, 24p can easily be converted for 25p/50i playback since the frame rate is only a frame (or 1.024 frames) off. 25p, however, is visually nearly identical to 24p.
*If shooting and editing for NTSC broadcast, which largely requires delivery in 60i, and the 24p look is still desired, shoot and edit and export in 24p and then telecine the resulting video file (adds 3:2 pulldown to convert to 60i).
