Something

If the icon is not required to allow the application to function (i.e.
can be substituted at run time and is not distributed with the
application) then the license does not apply. For example, this means
that users would not be violating the GPL if a proprietary application
uses a GPL icon theme.

However, if a non-GPL application references an icon name from
gnome-icon-theme, this should be considered as linking (as described
above).