I just accepted a job offer in St Gallen however am not sure where to live. In terms of cultural activities and gay life, is it better in St Gallen, or Winterthur or Zurich? Given, there is a major difference in terms of income tax payments and rent in each of these cities. What would you recommend? Would appreciate your feedback.
It might depend on where you are from and what permit you are given. I think with some permits you're limited to living in the canton where you work.
Just get a temporary place near to your work then make a judgement after visiting Winterthur and ZH. As blacbird says, if your work is a long way from the main station the commute could be crippling.
you could always rent a cheap flat in st.gallen for the weekdays and stay there tues-thurs and then "live" and enjoy life in Zurich. I'm in Geneva and if there is anything gay, I would go to Zurich (or Lausanne)
In terms of gay socialising (gay bars, clubs, meeting people, gay church, gay Christian house-groups, gay sports, gay health checks, gay home-nursing, gay or gay-friendly discussion groups): Zurich, Zurich, Zurich.
However, as others have said, it will be the permit that decides, for you, in which canton you are permitted to live.
If you are allowed to live the Canton of Zurich, I suggest checking the public transport maps, so that you can live somewhere with a good train route to Zurich HB (= main station).
Life ain't bad in S.Gallen, and to Zürich it's like 70/80 minutes with train. You could live in between next to a train station, also good for the rent you have to pay.