edit: sadly this one is not retroactive for some stupid reason. I've played 1300 hours of Gears, a good portion of that is PvP, and I didn't get this to pop upon starting the update.
FYI, the tracker does not work for this achievement. I just went into Social and finished a match on both Apex Lift and Mercy, and did not make any progress. You may have to keep notes on which maps you've played on unfortunately.
thanks to Devil Vergil XT for the tip:
"Go to "See all my achievements", click on GoW4, scroll all the way down and then you can see the actual tracker. I have 12% now. I did one in Casual AI versus and 2 in Insane AI Versus, so against AIs is also possible."
So, the tracker DOES work, but it still wont keep track of exactly which maps you've played on. So you still will have to keep a mental note, or an actual note, of which maps you've completed.
For reference, the maps are:
the hardest ones to find a match on will be Speyer, Impact Dark, Glory, and Reclaimed Windflare. You will eventually get this one on your way to the 100 Hours Played achievement. Good luck and go kick some ass.