"gigantic quantity" is a perfectly correct and usable phrase in written English.
You can use this phrase to emphasize a large amount of something. For example, "We harvested a huge crop this year, easily a gigantic quantity."
Natural resources - cotton, sugar, teak, rubber, minerals - were plundered in gigantic quantities.
They are also given out early in the evening before the gigantic quantities of wine can start to inflame.
Troops fighting in desert conditions also consume gigantic quantities of water, several quarts per day per soldier.
Then when the Icelandic volcano Eyjafjallajökull erupted in April 2010, throwing gigantic quantities of ash into the air and causing massive disruption to air travel, prices on BlaBlaCar skyrocketed.
So it would've been silly to be fussing over characters when gigantic quantities of text needed to be removed altogether.
Second, the Japanese central bank's pledge to buy gigantic quantities of bonds at home has buoyed asset prices elsewhere, including Italy.
Experts say that with such a huge number of individuals able to access gigantic quantities of secret documents, many of which should not have been classified in the first place, it is no wonder that leaking is becoming a growing problem.
Unlike crude oil, biomass is distributed evenly over the world and its quantity is gigantic, which makes biomass a promising energy source of the future.
Such decomposing blooms function as gigantic 'fertilizer heaps' releasing large quantities of carbon, nitrogen, and other nutrients for the benefit of nondiazotrophic and heterotrophic biota in the surrounding water bodies.
