I have been frustrated that my multiple workspaces feature has not been working under Ubuntu 12.04 and 12.10 using Gnome Shell with effects. Well there is a fix for this. Seems it is not enabled by default due to configuration in the latest linux kernel. The workspace settings are now controlled thru the Compiz Settings Manager.
First install the Compiz Settings Manager:
To install Compiz Config Setting Manager enter following command in Terminal:
sudo apt-get install compizconfig-settings-manager compiz-plugins-extra
In 'compizconfig-settings-manager,' this assumes you've installed it, which you'll need to, you'll have to select:
Category > General > General Options > Desktop Size
Then set "Number of Desktops" to "1."
Then right-click on Workspace-Switcher (lower right hand corner) and select preferences; and set 'columns' to the number of workspaces you want, and 'rows' to '1,' unless you want to stack them for some reason... After I did this Workspace-Switcher functioned normally.
Should be working ok now, there was an issue previously that was fixed right before or after release.
When you say "Gnome desktop" do you mean Classic or Classic (No effects)
The former uses compiz & the setting should be made in ccsm > General Options > Desktop size
The later uses metacity & can be set thru the WS switcher preferences & or in dconf-editor, path & key -