Is there sin upon one who mentioned the existence of homosexuality among students of the Grand Mosque, and who regrets his statement?

What you did involves two reprehensible acts: preventing the man from teaching his children the Quran in the Grand Mosque, and spreading the faults of Muslims, whereas you should have concealed them. The most objectionable aspect is that your motive was your eagerness to speak with him in English. So repent to Allah and seek His forgiveness, and do not repeat such an act.