For what it is worth, I have recently started a Gloranthan game here
in Berlin. Three players are Gloranthaphiles (including our own
Daniel Fahey), two don't really know the background. Th two new
players have gotten into the background and have actually contributed
to my understanding of Orlanthi mythology and religion.
The trick is, assume your players know nothing about the background
initially, and just explore it with them. Pretty soon, they will be
creating along with you.
Jeff