Hi, just to inform you about the result:
It didnt have anything to do with the gma-file, I simply used the same tool (that I also used for unlocking in Mapsource, provided in the first post) for unlocking the .gpi files separately, and now everything works perfectly!
By the way, the Unicode Version runs fine on my nuvi 1390, no problems so far.
Anyway, thanks for the help!