package gdm (not installed) failed to install/upgrade: subprocess installed post-removal script returned error exit status 1

Fun times updating from utopic to vivid. gdm didn't show any user list, so I tried to uninstall it; lightdm let me log in but then this error popped up a dozen times immediately after logging in with gnome-shell.

ProblemType: Package
DistroRelease: Ubuntu 15.04
Package: gdm (not installed)
ProcVersionSignature: Ubuntu 3.19.0-9.9-generic 3.19.1
Uname: Linux 3.19.0-9-generic x86_64
ApportVersion: 2.16.2-0ubuntu4
AptOrdering: gdm: Purge
Architecture: amd64
Date: Sat Mar 21 13:55:08 2015
DpkgHistoryLog:
 Start-Date: 2015-03-21 13:55:05
 Commandline: apt-get purge gdm
 Purge: gdm:amd64 (3.14.1-0ubuntu2)
DpkgTerminalLog:
 Removing gdm (3.14.1-0ubuntu2) ...
 Purging configuration files for gdm (3.14.1-0ubuntu2) ...
 rm: cannot remove ‘/var/lib/gdm/.gvfs’: Is a directory
 dpkg: error processing package gdm (--purge):
  subprocess installed post-removal script returned error exit status 1
DuplicateSignature: package:gdm:(not installed):subprocess installed post-removal script returned error exit status 1
ErrorMessage: subprocess installed post-removal script returned error exit status 1
InstallationDate: Installed on 2014-04-03 (352 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
RelatedPackageVersions:
 dpkg 1.17.24ubuntu1
 apt 1.0.9.3ubuntu1
SourcePackage: gdm
Title: package gdm (not installed) failed to install/upgrade: subprocess installed post-removal script returned error exit status 1
UpgradeStatus: Upgraded to vivid on 2015-03-21 (0 days ago)