Gnome and fvwm are mutually exclusive at the login menu.
I suspect the issue is that they each look for themes in a different place; assuming the themes are compatible, have you tried putting a link between the relevant folders and then selecting the theme once you are logged in?
The link isn’t working. Can you be a bit more explicit about what you are trying to do? AFAIK there should be no problem running Gnome programs within fvwm, that is, you can have Gnome programs appearing on the desktop - but you cannot simultaneously log in to two different desktops.
Your original question was confusing. As the link shows, you can use fvwm as an alternative window manager in either Gnome or KDE. This has nothing to do with themes but with replacing the default window manager. In each case, you load the session and change the default configuration of the desktop environment to use fvwm instead.
Depending on the applications you then use you may see little difference in the look and feel of the desktop environment or you may find some things display differently.