I'm the upstream maintainer for Sopwith. I'd like to point out that while Sopwith _can_ use GTK2 for its graphical interface, the preferred method as been SDL for several years. We had too many problems with GTK2 and deprecated it as a front-end. Sopwith does not have a hard dependency on GTK and so this recommended dependency can simply be removed from the Debian package.
There isn't any need for us to port to GTK3 because we haven't been maintaining the GTK2 port up to this point. The GTK build option can just be disabled downstream. Jesse
