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
You cannot play split screen on Red Dead Redemption: Undead Nightmare.
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
No, you do not need to be on the game you bought with Gmod to play it although you do need to have the game installed to run gmod and i also recommend having the game Half-Life 2 installed to get most of the textures and weapons and npcs and props that go along with it.
simply press E on the keyboard next to the cars door :) i play gmod add me on steam username is kayden