[gpm] Gnome 3.22 not locking desktop

[Narcis Garcia <informatica actiu net>]

I have a problem with Gnome 3.22 (default in Debian 9): screensaver is
not locking desktop.
I've reviewed privacy settings, with automatic locking enabled and timer
to 30 minutes. Screen goes to "blank" and monitor sleeps, but locking
feature has never effect. At "Energy" section blank screen is set to be
on 10 minutes.

What else can I ckeck?

Thanks.