If you enable Half-Life Source to be used with Gmod, then yes. However, this only allows you to use the maps and ragdolls. The NPCs (such as the Vortigaunt enemies and the HECU grunt enemies), weapons (such as the Gluon Gun) and certain entities (like the Health and Suit Chargers) cannot be used without the Half-Life Renaissance pack.
There's also a small glitch with the game in which some things (such as doors) appear invisible, so you are forced to "noclip" through them. This wouldn't be such a problem if it didn't happen so often.
"Program Files" or "Program Files (x86)," Steam\steamapps"Accountname"\garrysmod\garrysmod\maps
Not necessarily. If you own absolutely no Source games at all, then you can buy it, and you can then run Garry's Mod. A lot of Garry's Mod addons require Counter-Strike Source though, so it's recommended, not required.
for that you probably will have to buy?/rent a server. you can get one at the related links below.
the decision about what to include and exclude.
It depends on what you want to download, first get what you want to download off of garrysmod.org Maps go in the steam/steamapps/(username)/garrysmod/garrysmod/maps file addons go in the steam/steamapps/(username)/garrysmod/garrysmod/addons file
when you have downloaded an add on, extract the file to: :C/Program Files/steam/steamapps/<your user name>/garrysmod/garrysmod/addons. you put in maps to : :C/Program Files/steam/steamapps/<your user name>/garrysmod/garrysmod/maps.
Depends on what kind of things you want on your game for addons, and Maps get them off of GarrysMod.org Heres how you install them Program Files > Steam > SteamApps > YourUserName > GarrysMod > GarrysMod > Addons Program Files > Steam > SteamApps > YourUserName > GarrysMod > GarrysMod > Maps
Lake Michigan. Source: Google Maps
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You can keep multiple characters and maps.
Atlas
2,384 mi. Source: Google Maps.