This challenge is active from June 22nd to July 5th at 10am PDT.
You must win 50 rounds in multiplayer matches in either the 2v2 Gnashers Only limited time mode or in Competitive Versus Execution. Winning the match does not matter. Just 50 total rounds won across all matches.
You can track your current progress in the same spot as the achievements on the Xbox guide.
If you are having troubles finding matches searching by yourself (due to locale), you may want to partner up with another gamer from North America to speed up the process. LFG (Looking for group) on Xbox is an alternative to finding like-minded players.
In addition to unlocking the challenge you will also be able to redeem some weapon skins for your efforts. This challenge awards Flaming Gnasher and Heartbeat Gnasher. After unlocking the challenge, visit the below link and login with your Xbox account that unlocked the challenge and click 'Claim Reward' under the appropriate spot.
If you don't have the Flaming Gnasher you will get progress for the below achievement;
If you would like notifications for upcoming challenges or achievements for Gears 4, feel free to join the leaderboard I have made previously for the weapon skin achievements. The below information is posted over from my other solution.Gears 4 - Horde/Versus Weapon Skin Achievements
Any other questions, feel free to comment or shoot me a PM.