You can achieve this by adjusting the value in: Sensor Settings -> Customize -> Customize values -> Multiply
Multiplying the value by 3.785 should give the result in gallons. You can then also rename the unit to anything you like.
Here is what I did, but for some reason the Gadget still shows Water Flow. Should there be an icon on the far left? Lastly, I know my Gadget looks boring. I cant seem to figure out how to place a heading in between CPU, Graphics, Water Cooler, and Network. Thanks in advance for your help.
The HWiNFOMonitor is a bit outdated gadget and doesn't support features that were added later. Unfortunately it's no longer maintained and I don't have access to the source code because it was made by a different person.
You might look for some alternate solution like Rainmeter with HWiNFO plugin or the InfoPanel.