Translation:You are singing because you are cheerful.
Yeah, I know that – I just mentioned that, because with Polish huge catalogue of words with Germanic origin(German alone is 'responsible' for almost 30% of our words, and there are also numerous borrowings from Low German[during Hanseatic League period]; Old Norse/Swedish and so on), some might think the two are actual cognates, like say English 'thank'/German 'danken' and Polish „dziękować” or English 'rede'/German 'Rat' and Polish „rada”. ;)
Checking the English wiktionary for "gay", we find that such usage is "dated". Checking further what exactly they mean by 'dated', we see: "Formerly in common use, and still in occasional use, but now unfashionable; for example, wireless in the sense of "broadcast radio tuner", groovy, and gay in the sense of "bright" or "happy" are all dated. Dated is not as strong as archaic or obsolete."
So... why use it? I know this usage of course, but I really think it could be interpreted as "You sing because you are homosexual, singing is gay and not for real men"...