Note: The files don't change the Leaderboards. They are only for achievements!
Here my tutorial for hex the online achievements in Gears of War without any achievement unlocker.
1) Extract your profile.
2) Open your profile with Le Fluffie and extract the Gears of War GPD.
3) Open the GPD with GPD Entry Tool and extract TitleSpecific 1, 2, 3.
4) Open the files with Hex Workshop and make any changes. Use my bookmark to help.
5) Inject TitleSpecific 1, 2, 3 back in the GPD.
6) Inject the GPD back in your profile.
7) Rehash&Resign the profile.
8) Copy your profile back to your Device.
9) Start the game and play a ranked match. At the end of the match you get the achievements.
After each entry in my bookmark is a bracket with 4 bytes [00 00 00 00]. This means that those 4 bytes have to be before the actual entry.
If these 4 bytes are not before the entry, just look in another TitleSpecific until you have found it. But most of the entries are located in TitleSpecific2.