The first question to look at, is whether the boat is faster on one tack (many simply are) or whether you've got an instrument error. With currents and leeway, even using a GPS it can be hard to determine that. During slack, check and recheck to make sure it isn't instrument error.
And check that using the GPS versus the knotmeter, because often a knotmeter has been mounted offcenter so it reads differently on both tacks because it is "shallow" on one tack and "deep" on the other, and the water flowing past the different sides of the hull and keel is moving at different speeds.
There are also a surprising number of boats that have the keel put in a bit off center. (Ooops. Blush, Yes, don't look too closely down there.) You'd think C&C would be careful about that...but then again C&C's are generally lightly built and it is possible your keel was damaged, smacked a rock and shifted, or a PO recaulked it and didn't quite set it right--or FAIRED it wrong.
If you really want to perfect the keel, you get a full set of keel templates and fair and sand the keel until it is uniform and a perfectly symmetrical shape, while it is hauled.
Could also be a prop--is yours offcenter? That would pull the boatspeed on one side.
And then there's the rig, something may not be symmetrical and careful measuring with a steel tape measure (fiberglass sometimes stretches) from the masthead, make sure the mast is centered, check all the rigging, etc.
Then there's instrument error...If your masthead gear really is perfectly centered or not. That's a hard one, because there aren't many ways to make sure "IS this thing straight?" when you're up there. So I'd suggest looking into all the other stuff that literally is below that, and progressing up to the masthead last--even if that's a more likely place for the error.