Modding HL2:EP2

[Ceatra]

So hello guys

I want to make a mod for Half-Life 2: Episode 2, and that sould be a single/multiplayer zombie shooter open world survival game. Now I need you help cuz I don't know how to use the Hammer editor (I know how to use it, I just don't know how to make realisitc terrain, or half-realistic terrain (get that"-" half-life half-realistic? Huh, no? ),and I also want to know how to make drivable cars, how to make a inventory system where you can pickup and drop guns, items and that stuff, how to costumize (and dont know hot to write costume) the HUD or GUI, and how to add a food system.

Alot a stuff but for know I just need terrain, cars 'n guns

[Ceatra]

Don't you think Garry's Mod would be a better place to start?

Is it possible to make a mod with Garry's mod?