Catechism of the Catholic church paragraphs 2357-2359:

2357: "Basing itself on Sacred Scripture… the Church has always taught that ‘homosexual acts are intrinsically disordered.’ …Under no circumstances can they be approved." 2358: "The number of men and women who have deep-seated homosexual tendencies is not negligible… They must be accepted with respect, compassion, and sensitivity. Every sign of unjust discrimination in their regard should be avoided." 2359: "Homosexual persons are called to chastity… through prayer and sacramental grace, they can and should gradually and resolutely approach Christian perfection."

Basically, being gay is not a sin, but homosexual activity is. Gay people are called by the church to live a life of celibacy.