'gigantic mountains' is correct and usable in written English.
You could use it when referring to the size of mountains, for example: We drove past some truly gigantic mountains on vacation last summer.
In 1860, while carrying out a survey of Howe Sound on board the Royal Navy survey ship HMS Plumper, Captain George Henry Richards was impressed by a gigantic mountain dominating the view to the northeast.
But after 500 years of exploitation, the hill which, at almost 16,000 feet, is actually a gigantic mountain towering like a skyscraper above the ramshackle churches and plazas of this city of 240,000 people is as exhausted as its workers.
The eastern foothills south of Cantabria rise abruptly into the gigantic limestone mountains of the Europa Peaks, which include Torrecerredo (8,688 feet [2,648 metres]) and the Labra Peak (6,620 feet [2,018 metres]).
We built this mountain of empties in the corner, a gigantic pile of debris in the corner.
Less than 24 hours before, West seemed on top of the world, almost literally – rapping from the top of a gigantic manmade mountain.
However, if you don't live or work somewhere that is situated near a gigantic, illegal mountain of rubbish and waste, the shitting, vomiting insects are probably something you can tolerate.
"The views of lofty mountains, of gigantic trees, of falls of water … are indescribably unique and beautiful," the Times reported.
The sheer distance you can see in every direction and gigantic scale of the mountains and walls of ice.
Ralph Fiennes is on glorious form as Monsieur Gustave, the legendary concierge of the Grand Budapest Hotel in the early 1930s: a gigantic edifice in the mountains.
(AP) -- Stiff winds whipped up a gigantic blaze in the mountains of eastern Arizona on Monday, forcing the evacuation of a third resort town and casting a smoky haze over states as far away as Iowa.
On the morning of May 18, an earthquake with a magnitude of 5.1 on the Richter scale triggered a gigantic landslide on the mountain's north face.
