@peterph's answer was very close to the problem.
The video card was a Matrox G550 (mga) but in the past there been an nvidia installed and some glx stuff remained in
I've not experience with the mga driver but my understanding is that there's not a proprietary glx implementation, so we gone ahead trying to get the mesa working.
glx-alternative-mesa we checked the
ldd, then used
dpkg -S with full path to understand from which packages were picked the
libGLcore resolved by
ldd, just to check that was mesa, both the libraries weren't in any package.
Moved those libraries away and this time
ldd shown the right mesa libraries were used, at this point I asked @zar to check again and his answer been that this time
apt-get -f install finished properly and
gdm3 run without errors.
Even being a bug I think we can't fill any bug request as the non debianized nvidia driver broken the contract.
Proprietary drivers installation are still evil, I don't understand why they favor their own crap
supposed-to-install-everywhere.run instead of looking for some collaboration effort at least with major distros (that probably would come for free/no charge).