Yes, I also use it daily, so thanks for making an amazing tool!
100% vs 80% is of course going to be subjective, but the missing 20% in my opinion is built-in merging and a built-in repo browser. When I've tried to advocate GitUp to my coworkers, those are the two things that keep them going back to SourceTree or Fork.
(Also, on our (admittedly, pretty large) repos, it's not quite rock-solid; staging can be unusably slow (requiring fallback to the command line), switching to the commit view from the map view can be slow, and making lots of changes to the working tree will crash GitUp.)
Still, a brilliant piece of software, and I am suprised that (1) it didn't find a larger audience and (2) that it wasn't more influential on clients that came after, like Sublime Merge.
But yes, I wasn't really worth actively maintaining it as it was feature complete and I didn't intend to build a business out of it.