Half Life 3

[Toxic]

You're playing as Chell. Roleplay.
if that's what you call RPG than almost every game is RPG

Yeah, I'd call about every game I've played one form of RPG or another.

Portal 2 has ELEMENTS of an open world. Not a full-on open world as you're still slightly being pointed in the right direction. Details.
There are no decisions I can make in the way of going somewhere. It has one ending and I have no freedom to roam around anywhere but linear rooms and corridors. So tell me how do you see at least a single element of an open world?

http://8th-circuit.com/sites/default/files/portal2.jpg

Elements of an open world.

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A role-playing game (RPG and sometimes roleplaying game) is a game in which players assume the roles of characters in a fictional setting. Players take responsibility for acting out these roles within a narrative, either through literal acting, or through a process of structured decision-making or character development.

Yeah because that fits portal so well. just no...

You are playing as a character when you start any game.

Halo - Master Chief/Arbiter
Portal - Chell
Gears of War - Marcus
Half Life/ Half life 2 - Gordon Freeman

Want to go into decision making?

Walking Dead
Mass Effect 1-3
Spec Ops - The Line

Only difference on Garry's Mod RP is that your character's guidelines aren't set in stone.