A "gully washer" is Texas Redneck slang for a large, quick rainfall. It implies that a large amount of rain is going to fall in a short time so that the resulting run off will scour the gullies clean of trash, weeds and other flotsam.

Example usage:

Sam saw the dark clouds on the horizon and said, "Quick ma, get those clothes off the line, it's going to come a gully washer!"

Interestingly, there is a "Gully Washer" theme park ride at Six Flags Astroworld.