The only Gefen product I have ever used without problem:

DVI FM 1000 Extenders

The only issue with these are if you are outputting from a TH2G. If you do output from a TH2G then you will need a DVI DA on each of the TH2G output you wish to extend. Even still, use these things all the time and they work great.

A better pair (although I haven't tried them) are the DVI FM1500 DVI Extender with Recordable EDID

I would suggest those instead because they also have the recordable EDID but since I haven't tried them, I don't know how well they work.