Are you sure that's the size of your basemap [gmapbmap.img]? 966MB is huge. Maybe you have some other detail map image saved as the basemap, check what it is with [Only registered and activated users can see links. ]. You can see various basemaps here, scroll down to 'World and Basemaps' but don't choose a World map, they aren't basemaps.

A map of 1.85GB is under the FAT16 file size limit of 2,147,483,647 bytes whether that's 1.85 Gigabytes [decimal] or Gibibytes [binary] so the size isn't the problem. If you don't have a proper basemap then replace it and check your gmapsupp.img in GMAPTool as well to ensure it's not faulty and also unlocked [or just run [Only registered and activated users can see links. ] on it to be sure it's unlocked.

The name gmapsupp.img is ok with any version of GMPC, see here: [Only registered and activated users can see links. ].

My guess is you have a faulty map or it's duplicated by an earlier version named as gmapbmap.