That is indicative of a short between the sender side of the gauge and ground. Do the easy stuff first. I assume it returns to the low side when the key is switched off, and pegs when the key is on, right? Probably even before the engine is running right?
Try unhooking the connector to the sender. If it is still pegged, then it is in the wiring. If it stops pegging and returns to zero, then it is a shorted sender.