I recently beat the Half-Life series, and I really want to make a game myself, although I don’t have any more knowledge outside Lua, I know some C# (unity just doesn’t cut it for me). If I were to make this singleplayer gamemode, would people play it? Would they be willing to purchase it? Coderhire isn’t really a place to sell SINGLEPLAYER gamemodes, but if I were to make a good one, where would I sell it, and would people actually play it? In my entire GMod experience, I haven’t seen one singleplayer campaign-styled gamemode.
Unity is one of the most (If not THE most) powerful commercially available engines around. Its for proper games, not Garry’s Mod gamemodes. If it doesnt cut it for you, perhaps you are looking at the wrong thing.
You should spend some time mulling over your gamemode idea and posting a thread about your ideas so far. Campaign based gamemodes arent popular since Garry’s Mod is a community driven game based on multiplayer, plus it also has a single player counter part AND access to the Half Life story.