I'm using the following:
This is Freeciv version 3.1.90.3-dev (d8a54ddb6a), gui-gtk-3.22 client.
Built against gtk+ 3.24.29, using 3.24.29
Built against glib 2.68.2, using 2.68.2
Pressing + or - on the numpad or num row does nothing, and the Controls section of the help doesn't mention zoom at all.
Add zoom to the GTK client
Re: Add zoom to the GTK client
Oh, master development version. It's a bit different. Zoom can be enabled with --with-zoom configure option when building freeciv.