Does getting drunk qualify as grave matter?
Ordinarily speaking, yes, because it deprives a person of the use of his reasoning faculties and can open him to other grave actions he otherwise would not commit. Whether or not a particular person commits a mortal sin by becoming drunk depends on whether he has the requisite full knowledge and full and free consent. The Catechism of the Catholic Church says this about the use of alcohol:
The virtue of temperance disposes us to avoid every kind of excess: the abuse of food, alcohol, tobacco, or medicine. Those incur grave guilt who, by drunkenness or a love of speed, endanger their own and others’ safety on the road, at sea, or in the air
(CCC 2290; emphasis in original).