GPS or NO GPS in the footage, if someone want to use your footage against you ( if that is actually legal ) they will just use the video footage itself even if the cheap GPS in a dashcam are fairly accurate, then it is not a calibrated measuring devise, so i think no court will accept that as evidence.
But your video, well using that you can time how long it take for you to drive the distance from say "this" sign and to "that " tree, then go measure the distance in between the 2 and the time it took is in your video.
And you gat a very accurate math bases speed, which a court will accept.
Regarding the suction cup mount, many have used the universal mounts you can get for many things, like radar detectors ASO.
The problems with suction cups, at least in Denmark, you have to mount it fairly low, and then it will be in a place on the windscreen where it is illegal to install things.