"I have new underpants."
Translation:Mam nowe majtki.
And frankly, that's the answer. In plural, there isn't really a distinction between masculine, feminine and neuter, as there are only two plurals: masculine personal (groups with at least one man) and not masculine-personal (everything else). Obviously, with almost any plural noun, you can talk about its gender in singular. But only 'almost any'. As plurale tantum nouns simply do not have a singular version, they're neither masculine, feminine nor neuter, they're 'not masculine-personal plural'.
I have literally never heard that word, although apparently it's a thing, but it seems more like a sports thing, sold in sport shops. Would it really be translated to "underpants", though? I mean, the words look very alike, but wouldn't English have a different word for "podspodenki"?