Making it work for GTK3 is the minimum requirement, GTK2 is not supported by Geany any more, which presumably means upgrading the libdevhelp version (and hopefully to a system library, not embedded source) and adapting to any API changes that entails. And same for webkit2gtk-4.0 (noting that the markdown plugin has already been upgraded to use that).
Best to just make a PR, don't forget to handle new dependencies in CI or it won't build (markdown does not build on CI, which may be the webkit2 4.0 version not being installed).