Thank you very much for your kind and clear guide.
I'm trying to convert an img (NT).
First thing: the split result is aabout 22o GMP files!!
Second: each GMP contains only .TRE .RGN .LBL (missing, .NET, .NOD,).
Are these three enough for the further joining in GMAP Tool?
Is there a way to use Garmin-GMP-extractor with multiple GMP at once?