Originally Posted by MysterD

Civ 5 GOTY is a STEAMPLAY title - which mean it supports both Mac and Windows PC.
See that info on its Steam page.

You'd probably just take the key GameFly gives you; activate that key on Steam; and you're good to go.

EDIT:
When dealing w/ Steam-required titles, you want to see THIS symbol to see if your purchase will get you BOTH Windows & Mac PC versions.