Well of course if gay people can become straight through therapy, then the original question has a trivial answer: “Let them become straight. Then they can get married or become nuns, or do whatever.” However, according to NARTH, about 67% of those who attempt to change their sexual orientation will fail—not much difference between that number and 100%, is there? In any case, a large majority of gay Catholics will find themselves having to grapple with the issue of what they can do (since there is so much that they aren’t allowed to do).
Now, about the call to chastity, I do not dispute that each and every person on the face of the planet is called to this virtue. So I would certainly say, “Homosexuals are called to chastity… and so is everyone else.” I would could also say, “Everyone is called to chastity, therefore, so are homosexuals.” What I would not say is, “Homosexuals are called to chastity just as everyone else.” Why not? Because the phrase “call to chastity” describes a relation between the person, the ordinary elements of his or her life, and God’s intention in arranging things just so. It’s not as if the call to chastity were completely abstract, or known only by revelation. It is something that emerges in and through our lives. It follows that no one is called to chastity in precisely the same way. There is no contradiction here: just because Ms X and Ms Y are called to be nuns, it doesn’t mean they are called in the same way.
Why am I making such a fuss about this? First because it is misleading to say that homosexuals are called to chastity just like everyone else. It makes it appear as if it weren’t a special challenge to be gay and Catholic, when it plainly is. Most people can expect either marriage, or celibacy within a communal environment, but all that gay Catholics can expect is a lonely celibacy, ignored and unsupported (or even despised) by the community at large, both secular and Christian. Does this sound to you like the “call to chastity” that everyone else gets?
My other reason for not accepting the call to chastity as an answer to the OP is that it just isn’t an answer to the question. The OP asks, “What can a gay Catholic do?” Someone answers, “They are called to chastity.” The response doesn’t make any sense, because what the OP was on about was personal vocation for gay Catholics, and the response given named only a single facet which is present in the personal vocations of everyone who has ever existed. It’s like wanting to know what carpenters do, and being told simply that they make money.