Check the settings on the GPS. There's often an option for magnetic north or true north. Use magnetic north to check the accuracy of your compass.
The compass does not need to be on the centerline of the boat but the lubberline needs to be parallel with the boat's centerline.
You really need to know if your compass deviates 30 degrees from magnetic north at every heading or at just some headings. If it's every heading, you may be able to compensate with magnets or by moving some wiring or other metal objects away from the compass. If it the deviation varies dramatically with heading, there's not much you can do to fix it and your best bet is to make table of the deviation at 10 degree intervals or so. Less expensive compasses will have larger variations, but all magnentic compasses have some variation.
A fluxgate compass may be a great solution if you really care about headings. They are very accurate but don't come cheaply.
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