A description of size, meaning extremely large. Used in D&D to describe a creature between 'huge' and 'colossal'. In 3rd edition players handbook, pg 131, gargantuan includes kraken, purple worms, and 50 ft statues. To give an example more easy to understand, gargantuan is directly below, as already said, colossal, which is descriptive of creatures such as Red Wyrms(dragons). So something similar yet a small amount less than a Mature Red Dragon would be something of gargantuan size in D&D.