I don't think bytes.Buffer supports copying by simple assignment. Instead of returning a bytes.Buffer or even a *bytes.Buffer consider instead returning a []byte. Returning a []byte is more concise and less confusing because you are eagerly constructing the bytes in this function anyway. Also consider renaming linesToBuffer to linesToBytes or something like that.

I think what you're saying is that returning a bytes.Buffer is a form of copying by value and that the buffer might not end up containing the intended gzipped content. I haven't dug into that but I like the idea of returning the simpler []byte type - pushing that change now.

Slightly concerned about whether the refactor, although more readable, might cause any noticeable performance changes given we're creating a new buffer twice now (I don't know if we think of this code as performance sensitive or have any performance tests around it)

Regarding your concern about creating a buffer twice. The godocs for NewBuffer say: NewBuffer creates and initializes a new Buffer using buf as its initial contents. The new Buffer takes ownership of buf, and the caller should not use buf after this call. This makes me think that the new Buffer is using the existing slice for its storage which should be relatively cheap.