2005 Chevy Trailblazer gas gauge

It is either the sending unit in the tank or the gauge in the IP.

To check the sending unit you have to remove the tank and fuel pump module, connect an ohmeter to terminals 1 and 4 of the connector. Move float to full position and slowly move to empty position, if resistance goes down smoothly then sender is OK and problem is with gauge or wiring, if not then replace sender.