It's about 0.00000000000010 times as big as an Halley's Comet
In other words, 100 gallons is 0.000000000000094 times the size of an Halley's Comet, and the size of an Halley's Comet is 11,000,000,000,000 times that amount.(Comet Halley, officially "1P/Halley") (volume calculated based on 2005 dimensions)
Halley's Comet, the famous comet visible to observers on Earth approximately every 76 years, has a peanut- or potato-shaped nucleus made of rock, dust, ice, and various frozen gasses. The nucleus measures approximately 15 km by 8 km by 8 km, which yields a calculated volume of 1,100,000,000,000,000.000000000000000000 gallons. The coma of the Comet — the trail of sublimated gasses that give the Comet its visibility — may be up to 100,000 km in length.
It's about 0.00000000000009 times as big as The Grand Canyon
In other words, 100 gallons is 0.0000000000000908 times the size of The Grand Canyon, and the size of The Grand Canyon is 11,000,000,000,000 times that amount.(Coconino County and Mohave Counties, Arizona, near Fredonia and Grand Canyon, Arizona)
A dynamic and continuously changing landscape, the Grand Canyon has an approximate volume of 1,100,000,000,000,000.000000000000000000 gallons. The strata of the rock visible in the Canyon's walls display nearly 2 billion years of geological history.
It's about 0.0000000000000002 times as big as The Gulf of Mexico
In other words, 100 gallons is 0.0000000000000001555 times the size of The Gulf of Mexico, and the size of The Gulf of Mexico is 6,431,000,000,000,000.0000000000000000000 times that amount.(water volume)
The Gulf of Mexico contains 643,000,000,000,000,000.000000000000000000000 gallons of water. Every second, the Mississippi River empties 3,170,000 gallons of water into the Gulf.
It's about 0.0000000000000000003 times as big as Earth's Oceans
In other words, 100 gallons is 0.00000000000000000027 times the size of Earth's Oceans, and the size of Earth's Oceans is 3,700,000,000,000,000,000.00000000000000000000 times that amount.(Total water volume of Atlantic, Pacific, Indian, Arctic, and Southern Oceans) (estimated)
Over 97% of the Earth's water is found in the planet's five oceans for a total volume of about 370,000,000,000,000,000,000.00000000000000000000000 gallons. The Pacific Ocean is the largest ocean, covering 165,760,000 sq. km — more than double the area of the Atlantic Ocean at 82,400,000 sq. km.