I had the same struggle as well, but here’s what I do to stock up on GuK:
Shadowlands (try to get as many 6* on the team to get bonus rewards)
Alliance Raid (Medium/Hard mode) I often get 200 or 5* Urus from here which is SUPER helpful in saving materials.
World Boss Invasion. After completing all my daily runs, I spend the remainder of my energy here. It’s also a great source of spare bios, gold and norn stones.
Legendary battles, mostly you’d get gold and 1-2 bios here but there are chances to get GuK here.