Just bumped into this bug as well. As another person mentioned, the guile version bump requires a complete rewrite of GnuCash due to depreciated functions. I tried to patch configure.ac, but even after passing configure it fails to run properly. We'll likely have to wait on upstream. 

However, I took the time to make an ebuild for 2.6.18 which runs ok on my machine with after masking the new guile. Put ">=dev-scheme/guile-2.2" into your /etc/portage/package.mask and enjoy.