The spell doesn't currently exist, and I thought it should, so I tweaked the description for Form of the Dragon III and made it:
Form of the Dragon IV
School transmutation (polymorph); Level sorcerer/wizard 9
This spell functions as form of the dragon III save that it also allows you to take the form of a Gargantuan chromatic or metallic dragon. You gain the following abilities: a +20 size bonus to Strength, a +18 size bonus to Constitution, a +18 natural armor bonus, fly 250 feet (clumsy), blindsense 60 feet, darkvision 120 feet, a breath weapon, damage reduction 20/magic, frightful presence (DC equal to the DC for this spell), and immunity to one element (of the same type form of the dragon I grants resistance to). You also gain one bite (4d6), two claws (2d8), two wing attacks (2d6), and one tail slap attack (2d8). You can use the breath weapon as often as you like, but you must wait 1d4 rounds between uses. All breath weapons deal 24d6 points of damage and allow a Reflex save for half damage. Line breath weapons increase to 120-foot lines and cones increase to 60-foot cones.
As to why it's necessary: there are more and more Gargantuan dragon miniatures being produced and we GMs should get more use out of our expensive toys. ;)
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First, I'd advise against this. There is no way in the game (short of Mythic rules which I'm unfamiliar with) to polymorph into anything bigger than "Huge" sized. This is a balancing point of the game, IMO.
If you are insistent on doing it, at least cut the bonuses down a bit. By this table the size changes for large to huge and huge to gargantuan result in the same stat changes (save that you get 1 more point of natural armor for the later). If you're going to create the spell, use stats that are in line with the power of the spell, so increase the stats for Form of the Dragon IV by the same amount they increase from FotD II to FotD III - +14 Strength, +12 Con, +10 Natural Armor, Fly 150 feet. Likewise, the Breath weapon should be 16d8 and be 120 feet long for lines or 60 foot cones. Fly speed should be 150 feet (poor) and DR 15/Magic. Finally, the attack damages should scale by 1 size category each over Form of the Dragon III.
As meche stated, there is actually a mythic artifact called "tarnhelm", and does just that:
Allows you to turn into a gargantuan chromatic/metallic dragon
+14 Str, +12 Con, +10 NA
fly 150 ft. (Poor), blindsense 90 ft., darkvision 180 ft.
Breath weapon(16d8 every 1d4 rounds, Reflex half*), DR 15/magic, frightful presence**
Bite 4d6, 2 claws 2d8, 2 wings 2d6, tail slap 2d8
*DC 10+half tier+con because it'a mythic, houserule your own
**DC static because it's an artifact, houserule your own
Turns out it's exactly what meche said. Crap.