Apologies, I caused this confusion by mentioning both the existing setup under lightdm and what’s needed to work under GDM. The problem doesn’t lie with either display manager and the solution should apply to both.
In the past, under logind, Unity8 has not accessed input or graphics directly, but used a privileged
unity-system-compositor process to provide these through “Mir-on-Mir”. The problem alluded to in the linked article is that “Mir-on-Mir” is no longer supported because to do so would require additional distro patches to mesa.
The specific issue is that to access input as an unprivileged user shell (i.e. without
unity-system-compositor) Unity8 needs to request permission from logind. That’s a Mir enhancement (@raof) is working on.
I described this as “working with GDM” simply because that’s now the default on the desktop, not because GDM is a requirement.