--nogap is what is causing the extra "tmp" mp3 files. In my experience, this happened when another program (for example, EAC) called LAME to encode a WAV file that it had just extracted. This only happened to me when I was using EAC to call LAME and only when --nogap was enabled.

As for why there is no information on the --nogap parameter in the LAME documentation... I couldn't tell ya.