Dear Marc,
The maintainer's initial rôle would be to identify the version that seems the most appropriate (v4.7.?) and to create a git branch of that version. The most recent set of unit-tests should then be added to that branch.
This sounds like a great plan and I would be happy to help maintaining an older (e.g. 4.7.0-based) branch of Gambit. I would assume that you'd eventually host this branch in the official Gambit repository? We can provide a list of potential patches to it. Also by making this branch the default version in LambdaNative it will likely get a lot of use. However, I am not a compiler developer so there might be some limitations of what I can do with respect of troubleshooting problems with it.
For a start we (part-cw) could fork a 4.7.0 version and apply some patches against it to see what we'd like in a LTS version? It might be good to start an issue on Github as a central place to track the efforts needed to make this happen. Would you be happy with this?
The maintainer could also manage a branch created from a recent version of Gambit.
I don't think we can do this as we still use 4.7.9 internally and are very uncomfortable to go there due to the 1000+ changes you mentioned, most of which are unrelated to the C-backend we use. Hence, someone else would need to step up for this 4.8.5-based LTS version. I did, however, file an issue against it to point out one potential difference I noticed.
Many Thanks, Matthias