You are unable to play multiplayer on Red Dead Redemption without an internet connection.
No. Gmod is only for PC
You need to download Gmod 9.0.4c when you start there will be 3 choose unmark the Gmod 9.0.4b and your set to go and play gmod 9 online.
You cannot play multiplayer mode without internet connections.
NO
Garry's Mod (GMod) is primarily a sandbox game that requires the Steam platform for online play. While there is no official standalone online version, players can join multiplayer servers hosted by other users or create their own servers to play with friends. Additionally, some fan-made versions or similar sandbox experiences may exist online, but they are not officially affiliated with GMod.
You cannot play split screen on Red Dead Redemption: Undead Nightmare.
Yes, you need a Steam account to purchase and play Garry's Mod (GMod). The game is available exclusively on the Steam platform, where you can buy, download, and manage it. Additionally, having a Steam account allows you to access community features, mods, and multiplayer gameplay.
No you can not
gmod 9, yes gmod 10-no
yes the multiplayer works but the single player is only undead nightmare
No. Undead Nightmare standalone disc is the Undead Nightmare, Legends and Killers and Liars and Cheats packs (possibly Outlaws to the End too) + Multiplayer. There's a possibility your multiplayer lvl from RDR carries over when using the UN disc. I do not know as i have RDR and the UN download for Xbox but the UN disc for PS3