The Gryffindor Common Room is located in Gryffindor Tower. To enter the room, you need to give the right password to the guardian painting, the Fat Lady.
The Common Room is also the only way of getting to the Gryffindor dormitories, where the Gryffindor students sleep.
From the Story
"Students of Gryffindor house gather to socialise and study in the cosy, round, Gryffindor common room. Full of squashy armchairs and tables, the room is located behind a round stone hole that serves as an entrance to Gryffindor Tower. The entrance is guarded by a magical portrait - of a fat lady in a pink silk dress - which swings open whenever offered the correct password."
The entrance to the First Year Boys' dormitory
A table in the Common Room with a view of the Hogwarts grounds
The comfy seats by the Gryffindor common room fireplace
PlayStation®Home: Pottermore version of the Gryffindor common room
A Gryffindor student studying near one of the common room windows