Now to play it through again with the commentary and the gnome achievement...
Been there, done that.
Got all the achievements too which is not normally something I do in a game.
Tip: for a commentary/gnome replay put the game on story difficulty. Many enemies are 1 shots with the pistol, and you have loads of ammo, so it's far easier.
Also make sure you hold on to the gnome during transitions and always place it somewhere out of the way when going into combat. Unfortunately it has a habit of pinging off into the ether due to wonky physics, so you have to be careful how you carry and place it.