Growly is a low gain overdrive, paired with the Owly booster and order switch for versatility.
On the OD side, the gain isn't an external knob, but an internal trimmer. "Why?" You might ask, since it's once of the most used knobs on a drive pedal. The reason is simple, the focus is taken away from the gain knob and moved to the way you play. The drive circuit is very dynamic and sensitive to the guitar, pickups, the way you pick and the settings you've dialled in on the pedal. So what are the "bias" and "body" knobs for?
The "Body" is a variable input capacitor which controls how full your guitar tone is. Back it off to make the tone thinner and brighter, but also make the drive a little less. Turn it up and you get a fuller sound, giving more low end and low mids, as well as more gain. This can be used to compensate for different pickups which have different characteristics but also just shape the drive tone to your needs.
The "Bias" knob is a simple idea. Instead of having a switch that lets you switch between two clipping diode pairs, this one lets you dial them in. Turning it to the left dials in a pair of large germanium diodes for a more open and clear saturation. Turn it all the way to the right dials in a pair of small germanium + silicone diodes for a more compressed and driven tone. Set the "bias" in the middle and you have the least driven, but most open tone available. The "Body" and "Bias" knobs act together to shape the drive quite a lot and depending on the guitar and amp you use, the outcome can vary significantly.
The boost side is the "Owly" booster, unaltered, and can be used on it's own or together with the overdrive side. Use the switch in the middle to set wether you're boosting the signal into the drive, or boost the volume of the driven tone. The order switch (in the middle) points in the direction of the signal flow. Pointing to the left means that the booster is going into the overdrive. Pointing to the right meanest that the overdrive is going into the booster.