Just do the math. If they are 6GB with 12 left to play, 6-6 would require the Cubs to go 12-0.
They could go 9-3 and the Brewers would stay in 1st by going 4-8. The Brewers have been middling. Not atrocious. They've basically been what the Cubs have been, thus the needle not moving at all for 3 weeks in either direction.