Here are some synonyms for "gargantuan," depending on the specific nuance you want to convey:

General Synonyms:

* Colossal

* Enormous

* Gigantic

* Immense

* Massive

* Monumental

* Vast

* Tremendous

* Huge

* Giant

More Specific Synonyms:

* Brobdingnagian: (emphasizing extreme size)

* Cyclopean: (referring to something of immense size and strength)

* Herculean: (referring to something of great size and power)

* Titanic: (referring to something of extraordinary size and power)

* Prodigious: (referring to something exceptionally large or impressive)

* Whale-sized: (used to describe something extremely large)

The best synonym to use will depend on the context of your writing.