Individual acts of overeating…intemperance (gluttony in this sense) are “ordinarily” a “venial matter” for venial sin…
(though gluttony can be grave such as when one is getting drunk where one looses ones reason is mortal for example (or if it leads to other mortal sins)…or uncorrected habits of eating in a way that will “seriously harm ones health or life” --at least in the “short term” that is…)
We need to seek to grow in the virtue of temperance…
1806 Prudence is the virtue that disposes practical reason to discern our true good in every circumstance and to choose the right means of achieving it; "the prudent man looks where he is going."65 "Keep sane and sober for your prayers."66 Prudence is “right reason in action,” writes St. Thomas Aquinas, following Aristotle.67 It is not to be confused with timidity or fear, nor with duplicity or dissimulation. It is called auriga virtutum (the charioteer of the virtues); it guides the other virtues by setting rule and measure. It is prudence that immediately guides the judgment of conscience. The prudent man determines and directs his conduct in accordance with this judgment. With the help of this virtue we apply moral principles to particular cases without error and overcome doubts about the good to achieve and the evil to avoid.
1809 Temperance is the moral virtue that moderates the attraction of pleasures and provides balance in the use of created goods. It ensures the will’s mastery over instincts and keeps desires within the limits of what is honorable. The temperate person directs the sensitive appetites toward what is good and maintains a healthy discretion: "Do not follow your inclination and strength, walking according to the desires of your heart."72 Temperance is often praised in the Old Testament: "Do not follow your base desires, but restrain your appetites."73 In the New Testament it is called “moderation” or “sobriety.” We ought "to live sober, upright, and godly lives in this world."74
To live well is nothing other than to love God with all one’s heart, with all one’s soul and with all one’s efforts; from this it comes about that love is kept whole and uncorrupted (through temperance). No misfortune can disturb it (and this is fortitude). It obeys only [God] (and this is justice), and is careful in discerning things, so as not to be surprised by deceit or trickery (and this is prudence).75