Making GWTD compatible with GWT 2.6 and beyond is certainly possible. I suppose that depends on what exactly is wrong with it. If it is just a matter of patching the problem described in
Issue 8558, and rebuilding it as-is, we should be able to make that happen (although it sounds like you are restoring the old API that GWTD needed, so is that really needed?).
As far as on-going maintenance is concerned, we could certainly add new committers to the project and turn them loose. The existing committers from Google are all working on other projects and have very limited time for anything GWTD-related. There are a couple of non-Google committers (also working on other projects) who might be able to do some work on it, but I can't speak for them. If anyone wants to be added as a committer to GWTD, send me a note.
The biggest challenges with GWTD are actually unrelated to GWT and any changes there. Keeping GWTD updated and working with new GWT versions was always fairly straightforward. The hardest problem was dealing with OS changes (almost always on the Linux side) that broke it. Every time a window manager or distro changed, it would seem to break GWTD. Incompatibilities between Linux distros, versions, and window managers was a very special kind of Hell I never want to revisit. Working on GWTD made me (and the other committers) really hate Linux for a long time (and I still have a bad taste in my mouth from it).
The other major issue is the build environment. The current build is unfortunately tied into internal Google infrastructure. GWTD really needs a completely new build based on the latest standard Eclipse build tools. Creating that will probably take someone a several days to a week of work. I don't see any help coming from our end on doing that. Even sanitizing the current build system enough to dump it out there unsupported (and definitely non working) would take some time from folks I can't speak for.
So... if someone wants to take it over, we can help make that happen (and try to answer questions as they come up).