The Katana should be recognised and appear as 4 channels with a MIDI.
Takashi Iwai spent a fair bit of effort, a few years ago, trying to get the Roland/Boss kit working with ALSA. I did a lot of testing of his patches with a bunch of Roland/Boss kit to support that, including a MK1 Katana, a GT-1, a JS-8, a GT-001 and some others: [PATCH 00/41] USB audio refactoring for better implicit feedback support - Alsa-devel - mailman.alsa-project.org
As @Schmitty2005 suggests, the MIDI is for programming with the app. You could, potentially, use this to turn on and off effects on the amp by putting the appropriate MIDI data into a MIDI track, it I’m not sure why you would want to do this. It’s probably best ignored.
As I said, the Katana presents 4 audio channels (4 in and 4 out). These are used as follows:
Capture channels 1 & 2 are stereo left/right of the effected signal. This is what you would normally record.
Capture channels 3 & 4 are the “dry” (uneffected) guitar sound. It’s a stereo pair but, due to no effects being applied, both channels are the same.
Some people like to record this so they can change the effects at a later date.
Playback channels 1 & 2 are the “dry” playback, so no effects are added to the signal played back here. Note that this signal does not play back on the Katana speaker, but can be heard through headphones.
Playback channels 3 & 4 are the “wet” playback, so effects are applied. You can use this, for instance, to re-amp the dry signal.
My suggestion is to use ALSA. You should then be able to create a stereo track with inputs 1 and 2 to record
Connect the master out to outputs 1 & 2 and you should be able to hear the playback through headphones connected to the Katana.