Geary depends on webkit-gtk:2 which is a vulnerable version, for more info refer to bug 577068.
Lets see when upstream releases a new version based on 'master'
Created attachment 499990 [details] Updated ebuild for 0.12.0 Ebuild for v0.12.0 released on October 2nd, 2017.
Created attachment 499992 [details, diff] Updated vapigen environment variable patch The previous vapigen patch for 0.5.3 doesn't apply cleanly, this is an updated version that works on 0.12.0
Upstream has released a new version, that depends on webkit-gtk:4
[master 1a5dcae6e6a] mail-client/geary: Version bump, also now supporting webkitgtk4 2 files changed, 79 insertions(+), 1 deletion(-) rewrite mail-client/geary/Manifest (100%) create mode 100644 mail-client/geary/geary-0.12.0.ebuild