https://issues.asterisk.org/jira/browse/ASTERISK-17454 seems to suggest version 1.8.5 will have gmime-2.4 support. Blocking #311497 for now.
1.8.5.0 in CVS; permission to commit for this ebuild (without revision bump).
+ 12 Jul 2011; Eray Aslan <eras@gentoo.org> asterisk-1.8.5.0.ebuild: + Support gmime-2.4 - bug #373505 +
Commit has been made; 1.8.5.0-r2 still carries it.