"Since GSnap is mono-only, it mixes the left and right channels of any stereo signal before processing and outputs the result to both channels. The result in this case is that the signal is mixed with silence and you lose half of the signal.
GSnap supports 1x1 processing, so VST hosts generally use that when processing a mono signal.
Workarounds:
1. As you suggest, you can boost the signal back up. It will need 6.02dB or 200% (assuming all of the above is correct). Depending on how Audacity deals with things internally, you may lose some dynamic range in the process. However, I think Audacity sticks to 32-bit floating point internally so you
2. Duplicate the track and make a stereo pair, process that, then split it back to mono. This way you should retain the full dynamic range as the signal hasn't been squashed and stretched again." (Link).
