I thought it was called the gap because it was the only remaining part of I-25 between Denver and CS that was 4 lanes.
This is how I've heard of it, and only after CDOT started using it for the
"Mind the Gap" tagline and studies. Apparently the Pikes Peak Area Council of Governments
used it a bit earlier in a resolution and priority name, though. Maybe someone else before?
It was a pleasant surprise for things to open up early--a nicer mirror to when they pushed expected completion back from '21 to '22. Nice to have the speed limit raised (mostly to 70mph) to closer to traffic speeds since the lanes were put in their final alignment (well above 70 whenever possible).
I've been through each way in the express lane once. In both directions, I was in behind someone going about the speed I wanted to go, which was comfortable for me, but we picked up one car each way that wanted to go faster; southbound, that car entered and exited the lanes over the double white line. I feel like that could be typical of these, maybe even more than the Denver-area express lanes, where it's a relatively long distance to be caught behind or in front of someone in the express lane, or where something holds back the GPLs (like when you get a slow passing or people driving next to each other for miles).