All of the comments on this post that include numbers show that using both is better than just one or the other, mostly when the code’s sufficiently large.
I agree with the recommendation of one good compression algorithm vs combining multiple in general, but is it possible that you hadn’t considered that this is a case where we have a very good lossless compression (gzip) that can get benefits when combined with a lossy compression (minification)? As wonderful as Huffman codings can be, it’s hard to compete with outright throwing “unnecessary” (to many users, maybe not OP and some of us) data away.