Of course it can be of grave matter. Any of the 7 deadly sins can be of grave matter and can qualify as mortal sin. To put it into perspective for you, Pride is ranked as the worst of the 7 deadly sins, while lust is the least serious. Gluttony is somewhere in between.
Where did you get that order from? Just curious.
Dante placed gluttony second after lust, and pride is the worst.
Lust
Gluttony
Greed
Sloth
Anger
Envy
Pride
From Wikipedia:
Ranked in order (worst sins listed last) like in Dante’s Divine Comedy (in the Purgatorio), the seven deadly sins are:
Lust (fornication) — Unlawful sexual desire, such as desiring sex with a person outside marriage. (Dante’s definition was “excessive love of others,” and this reduced the love that a person could give God).
Gluttony — Wasting of food, either through eating too much food, drink or drugs, misplaced desire for food for its taste, or not giving food to the needy (“excessive love of pleasure” was Dante’s definition).
Greed (covetousness, avarice) — Greed is when somebody wants more things than the person needs or can use. Dante wrote that greed is too much “love of money and power”.
Sloth (also accidie, acedia) — Laziness; idleness and wastefulness of time that a person has. Laziness is hated because:
Others have to work harder
Delaying what God wants a person to do or not doing it at all
It makes life harder for oneself, because useful work does not get done
It, like gluttony, is a sin of waste, for it wastes time, maybe because of pride
Sloth is a state of equilibrium: one does not produce much, but one does not need much either (in Dante’s theology, sloth is the “failure to love God with all one’s heart, all one’s mind, and all one’s soul”; specific examples including being lazy, being scared, lack of imagination, complacency, and not doing what the person should do).
Wrath (anger, hate) — Inappropriate (not right) feelings of hatred, revenge or even denial, as well as punitive desires outside of justice (Dante’s description was “love of justice perverted to revenge and spite”).
Envy (jealousy) — Hating other people for what they have. Dante wrote that envy is “Love of one’s own good perverted to a desire to deprive other men of theirs” (in other words, thinking that the person himself should have more, even if it means someone else will have less because of him.)
Pride (vanity) — A desire to be important or attractive to others or excessive love of self (holding self out of proper position toward God or fellows; Dante’s definition was “love of self perverted to hatred and contempt for one’s neighbor”).
