Currently in linux there's no specific handling for the Forte, and its controls aren't exposed for automatic discovery. It behaves as a 2 in 4 out device with no controls, and uses its internal default settings for the mixer matrix. This mixes the inputs and the 4 outputs down to 2 channels and sends them to both the lne and headphone outputs. The inputs are set to line rather than mic or instrument and gain is fixed. The output levels for both line and headphone are controlled with the knob. It's no good for measurement, but works quite well if you've got a usb source (squeezelite in my case) and analog source.
Spurred by recent developments on the mixer code for the Gen 2 Scarlett, I started looking at the Forte last week. The USB product id is in the middle of the Scarlett Gen 1 models. A quick look at some usb captures showed similarities to the usb messages used by the existing scarlett gen 1 linux mixer code, so I tried adding a config block for the Forte. The function that adds the controls for the mixer matrix needed changing as the Forte has a smaller matrix, but after that alsamixer has controls, at least some of which work. However something causes it to stop responding after a short time. Based on more detailed inspection of usb captures from the Windows driver I've found differences, particularly around the input controls, that may be part of the problem. I'm new to the internals of alsa and usb, so I've got some learning to do, but I'm hopeful at least some of it will work.
The existing Scarlett Gen 1 mixer code includes the 18i20, but it looks like it only covers the mixer matrix and outputs as the code for the input controls is commented. A comment says the 6i6 and 8i6 are untested too. If there are bits of it that don't actually work it may have a bearing on what I'm trying to with the Forte.
I'll leave it there as it's got somewhat off topic. If anyone's interested in the details I can start another thread for it.