I bought a second GSC-10 (but perhaps should have got something different, I'll come back to this later). I have managed to get it working OK with a strong magnet from an old Hard Disk Drive, which is simply stuck to the flywheel at the base of the cassette (no need to remove the flywheel cover). I did try stacking a few magnets, but the magnetic field was too strong and when in the granny gear, the magnetic force tried to pull the chain off! just a single magnet works fine to get speed.
I have two minor issues, otherwise I think this combo is awesome.
1) because I have two GSC-10 speed/cadence sensors, trainer road appears that it can only pick up one at a time, so I configured TR pick-up the flywheel speed and hence give me access to virtual power. I have my Edge 500 unit on, which picks up the other for cadence, so I can do cadence drills OK, although it would be nice to have speed (power) & cadence in TR. Perhaps I should have got a speed only pickup like the new magnetless garmin one, as i reckon TR would have been able to pickup the GSC-10 as cadence and the new one for speed.
2) The variable magnetic resistance of the D2R and the corresponding power curves means you must be diligent as to what "click" setting the trainer is on. I am currently using the '8 clicks' curve, although it can be a bit difficult to ensure you are at exactly 8 and not 7 or 9. I usually have to crank it hard to the softest setting and count clicks, repeat a few times so i am confident i got it in the right spot. I suppose it doesn't matter so much which one you have it in as long as you are consistent (and assuming you are only interested in your relative performance from one session to another and don't care for absolute accuracy. I did 2 FTP tests in the first 2 sessions and came back with dramatically different results, I am convinced I perhaps had the D2r on 9 clicks the first session and then 8 for the subsequent. I will be upset if it is the other way around i.e. was 8 in first test and 7 in subsequent, as i will have taken a hit of over 50W on my FTP ;-)