How to use every single font Ubuntu installs by default?

[Climber]

I discovered a weird problem:
Although a freshly installed Ubuntu has already put them
in /usr/X11R6/lib/X11/fonts and according subdirectories, some fonts,
like Helvetica, cannot be selected in the GNOME font settings. You can
easily confirm their presence by just looking into the said folders.
Does anybody know how to activate this fonts for system-wide
accessibility?

Sincerely yours,
Climber