You have options:
1) Naming convention allows for more then one name. Allowed names are:
GMAPBMAP.IMG - base map (not routable AFAIK use it for base map only)
GMAPPROM.IMG - first detailed map
GMAPSUPP.IMG - second detailed map
GMAPSUP2.IMG - third detailed map
1) Gmapbmap.img Basemap
2) gmapprom.img gmapprom.unl rename your map.
3) gmapsupp.img gmapsupp.unl
4) gmapsup2.img gmapsup2.unl
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