When i first installed it from https://extensions.gnome.org there was a bug: the Grid (in Activities) wasn't styled anymore. and the wallpaper was only on one monitor. I disabled/enabled gTile and now it looks fine.
Looks like an init bug. Maybe I reset something when i disabled an extension.
Gnome Shell 3.4.1 Multiscreen Extensions that could interfere(?): gTile Workspace Grid Impatience WindowNavigator