Well, it's almost a month since I first post. As of now, I still have no idea how to detect when iG gives instruction. iNav make it so freaking difficult to detect the event. I kind of don't want to go the route that iGmon did by comparing sound level. In the mean time, the best option I could do is detect when Guidance screen come up then execute GPS command.
Hopefully I'll have it done by holiday travel time (X-mas) or whenever next offical RoadRunner release.