Mavericks is not yet supported it seems - "Incompatible Plug-ins Disabled [..] Contact the makers of these plug-ins for versions that are compatible with Mail 7.0 and Message 7.0."
https://s3.amazonaws.com/gpgtools/GPGMail-Mavericks-P2-hack....
And you need this fix for the latest DP 4: http://support.gpgtools.org/discussions/everything/9888-mave...
In addition, as the apps are linked and compiled with different settings (clang, no ppc, no 32-bit, etc.) then plugins that are dependent on these APIs have to find their own way.
ML was a huge change from Lion in many ways. Mavericks has some issues that an individual can resolve on their own (mostly) but it's a moving target as it is not released yet.
But I do encourage you to donate. Integration with your native mail app should be worth $50-100+.