To troubleshoot, you should check the instrument panel fuses (often located in the underhood fuse box and/or the interior fuse panel, usually under the dash). Look for fuses related to the instrument panel, gauges, or body control module (BCM). These fuses are more likely to affect the gas gauge's function than a dedicated "gas gauge" fuse.
Your owner's manual is the best resource. It will have diagrams of your fuse boxes showing which fuse controls what. If you can't find the problem with the fuses, it's best to take your Malibu to a qualified mechanic for diagnosis.
