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You won't go over your VOLTAGE, it's the amps that count. 12V is still 12V. Figure up how many amps the HIDs use, see what the GPS current rating is and add them all up.
Why do you want to connect it to your headlight circuit? The HIDs are a strain on the system as it is. There are other, and better, 12v sources to tap into.
What wattage are the HID bulbs and ballasts - 35, 55 or what? You can calculate the current draw (amps) once you know the wattage using the formula I posted previously. The GPS should say how many amps it uses somewhere on it.