Question if Quixel/Epic answers, but currently the Gloss layer works in all parts of Mixer, but is not displayed in the right-panel for manipulation:
Given we can pull assets with this information, isolate it in the viewport, it would be consistent with the rest of the UI/experience to allow the Gloss layer to be interacted-with on the right-panel, like the rest of the layers. Not having it is like false-hope in that we’re getting the stuff, but I cannot change the intensity, turn it off, etc.
For my part, Mixer as it stands otherwise works very well for me! However, my understanding is best-practice is to push-back any tweaks/adjustments in a material, back to the texture so I can rip out that part of the math (bake it back2texture for more efficient shader). I cannot do this currently as Gloss is not available on the right-panel… 
Moreso, most assets don’t have any information in the Specular channel(!), which IS available on the right, but being empty, basically worthless… 
Double-dinged.
I know Mixer is on/headed-to the dung-heap, but this seems like a somewhat serious omission in that one cannot get-all-the-information-out-of-the-app.