Tacx Videos come in two flavours;
RLV - where the video playback needs to adjust to turbo speed (.avi, .rlv, .pgmf)
ErgoVideo - where the load is synchronised to the video (.avi and .pgmf)
We support ErgoVideos because, essentially, they are the same as a Sufferfest video and an erg file. I own and have used the Milram and Cadel Evans ergovideos. We will eventually add video playback control for RLVs, but its not high on the list at present.
So in summary; ErgoVideos work with GC, RLVs do not.