It's just serial data. Nether the GPS or the Radio know what brand of box the data comes from. Garmin says they don't support it because it's not a Garmin radio. Tell them you have an SL-40. They'll support it--it's a supported function. If you have many electronics, you'll be sending the same serial data from your GPS to the EFIS, A210, Autopilot, etc.
Besides the rant, I don't have firsthand knowledge yet. I'll be wiring my panel soon, so if you you still need help in a few weeks, PM me.