Do you know what KMS aka kernel mode setting really is? It's about setting screen res and depth. It's more a security-thing than something relevant for graphics. Guess why the OpenBSD guys are happy about it ;-)
TTM is a memory manager for graphic cards. And GEM is the new memory manager for graphic cards, replacing TTM.
GEM:
http://keithp.com/blogs/gem_update/
So I really don't know what you're referring to ...
Maybe you're talking of Gallium3D, well ... that's a different story. But it's barely usable at the moment. Using Gallium3D requires working GEM in the OS. So yes, GEM is a necessity for the (near) future.
Finally there is no such thing as GEM/TTM/KMS, there is just a need for KMS and especially GEM.