The original map can be found in the Half-Life 2: Episode Two mod 'Mission Improbable'.

This particular map is a modified version for use in Garry's Mod, named 'rp_coastalimprobable'.

Mission Improbable mod for Half-Life 2: Episode Two - Mod DB
http://www.moddb.com/mods/mission-imp...

Steam Workshop :: rp_coastalimprobable [v2]
http://steamcommunity.com/sharedfiles...