I love Gullfoss and use it often, but am new to Kemper. My thought is...
Gullfoss is an EQ that provides that final 300+ band EQ driven by an algorithm, that gives a final EQ polish to a track. Gullfoss doesn't know what is playing through it, it's just sound. So it looks for those freqs that are offputting and tames them, or fundamentals that are missing and recovers them. That's my take at it anyway lol
Because the Kemper DOES know what is playing through it (guitar, acoustic, or bass), along with a profile that the input sound is running through, as well as a bunch of other parameter settings... Power Cabinet's algorithm theoretically could impact the sound much further than just an EQ.
My thoughts anyway reading about Power Cabinet without any practical experience yet