I have a couple of GR50's and had attached one to my 999. I've used these on my 798ci HD's for years. Being round the same generation, never thought it would be a problem. It caused very erratic behavior, see video (opps won't take a MOV file). On the GPS diagnostic screen, you would see all the birds, then the screen would flash a few times and lose track of all or most of them. HB support confirmed it was no compatable. The AS GRP works fine.
Just thought I'd post if someone else was working with older HB stuff.
BTW, it would do this if the setting for gps was internal, external, or auto. My suspicion is the GR50 draws too much current from the 999.