'gigantic mountain' is correct and usable in written English.
You can use it to describe a mountain that is disproportionately large. For example, "The view of the gigantic mountain before us was breathtaking."
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.
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.
We built this mountain of empties in the corner, a gigantic pile of debris in the corner.
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]).
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.
Twice the size of its New York counterpart, the Parque is a gigantic green finger of mountain that reaches down into the city from the Andes.
Once seen, his apocalyptic visions — of deserted megalopolises, gigantic dams in dizzying mountain settings, and abandoned cities, half destroyed, flooded or disintegrating and disappearing beneath the slow-motion onslaught of forest and vegetation — are not easily forgotten.
"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.
