Turning a gloss map into a roughness map is as simple as inverting the channel. The fancy combined maps that HDRP uses aren’t that complex either. It’s just a matter of combining channels into a single image. While these things are technically possible in Blender i would suggest using a dedicated material program for this, such as Substance
As for your last question, Unity seems to have gone knees deep with Autodesk partnership, and blender is a competitor to Autodesk.