gmap is pc install files,
copy it in:
(in XP - C:\Documents and Settings\All Users\Application Data\GARMIN\Maps) or
(in Vista, Windows 7/8 - C:\ProgramData\Garmin\Maps)
(in MAC OS
/Library/Application Support/Garmin/Maps/)
You'll have your map in Basecamp/MapInstall.
After that you can load the .img in etrex with Mapinstall