Summary says it all. Another thing is, that the ebuild has no code to activate or deactivate the external gd support. It just depends on it.
I fixed the mismatch as part of routine ebuild cleanup, but I'll leave this bug open because of Sven's activation/deactivation comment
That should be fixed now - plplot always used external gd, so I have corrected the deps accordingly. Thanks for pointing out the error.