Yeah there are a few manual steps to perform before running GIMP for the first time on Windows. One of them is generating GLib schemas. We used to have an additional job doing these, but I removed it because now we were providing a weekly unstable installer (last one was made not even a day ago), and I figured that anyone would prefer a proper installer rather rather than some zip file containing random files with no desktop integration whatsoever.
How to find the last version is actually explained on the devel download page, under the section in the “Automatic development builds” subsection.
It is not so obvious to find unless you read quite a bit so I expect many people don’t know. Unfortunately we don’t have a way to just provide an easy single link (I have a bug report/feature request to Gitlab about this, so that we could have special easy link in the future). On the other hand, it’s anyway very experimental code. It might be code in the middle of big changes or whatnot. So we prefer that only people who are really into testing experimental versions to help developers get access anyway. In this light, it’s maybe not a bad idea that it’s not too easy to get (we would not want someone to run this like they run stable versions).