I'm not so sure that gmapprom2.img would be acceptable for a nüvi as mentioned in the previous post. Try gmapprom1.img or gmapsupp.img instead.
SUM files are just check-sum data calculated when the unit uses the diagnostics mode.

The normal map IMG files to use:
  • gmapoem.img - Mostly for OEM data with customized POIs for the OEM reseller
  • gmapbmap.img - Basemap
  • gmapprom.img - Preinstalled map
  • gmapprom1.img - Second preinstalled map
  • gmapsupp.img - Supplementary map
  • gmapsup2.img - Second supplementary map
  • gmaptz.img - Time zones
  • gmap3d.img - 3D buildings primary map (only compatible units)
  • gmap3d1.img - 3D buildings secondary map (only compatible units)