If you complete the task indicated below then you will unlock the weapon or skin listed. If you earn an unlockable related to an Achievement, it won't be unlocked until the day after though.

All Gold Weapons (including Gold Gnasher) - Earn the "Veteran Gear" achievement in Gears of War 2.
Chrome Gnasher - Finish the campaign for all three Gears Of War games.
Chrome Hammerburst - Earn the "Dirty Dirty Horde" achievement in Gears Of War 2.
Chrome Lancer - Earn the "Don't Hate The Player" achievement in Gears Of War.
Chrome Retro Lancer - Earn the "Domination" achievement in Gears Of War.
Chrome Sawed-off - Earn any achievement for Gears Of War PC.
Gold Lancer/Hammerburst - Play during the first week of the game's launch.
Gold Omen Skin Set - Achieve Rank 100 in Gears Of War 3
Gold Sawed-off - Earn the "Commando" achievement in Gears Of War.
Liquid Metal Skin Set - Purchase the Gears Of War 3 Season Pass.
Onyx Skins - Earn the Onyx medal for a weapon to unlock the corresponding skin.