I doubt drop is from gpu, most likely cloud.
Big cloud miners tend to mine new coins at the coin beginning and not stay for long. There is few newer cpu coins so maybe they moved further.
I thought ZETTEL coin is a CPU efficient and it's faster than GPU.
It is definitely GPU resistant. Also pool resistant for now. When some of those become available network hashrate will skyrocket.
Not sure if it is GPU resistant, the dev did say some time ago, when the hashrate shot up, that a GPU miner may be mining Zettel.
If someone is mining with gpu hashrate would be huge, keccak hashrate for one 1060 is 360 MH/s, one 1070 is 685 MH/s, one 1080 is 900MH/s.