is it still actively maintained?
More or less, it hasn't existed for a long time yet. There used to exist a TGUI.Net port for tgui 0.6 several years ago, but it was discontinued because I didn't have the time to keep it up to date with the big rewrites going on in the c++ version. Now that the api of tgui 0.8 is getting more stable, I created the current TGUI.Net binding.
It is only since last weekend that the builds have been automated so that any change I make to TGUI.Net is downloadable on the website within a few minutes. Before that, the last (and only) release that was made dated from February. I expect to be updating it more regularly now, to only keep it at most a few commits behind on the c++ version. And the more people using TGUI.Net, the more incentive I will also have to keep the .Net version up to date.