Spent a few hours wiping out harpies on level 1 and 2 of Covetous and haven't seen that guy once. Just wondering if he just takes longer than other bosses to spawn...?
In my experience, if you're waiting for Gamayun to spawn and have plenty of regs/hp then he will never spawn. After you've killed that 100th harpy, are low on regs/hp and a dozen reds are around the corner with FS precasted, Gamayun will spawn.
I managed to spawn him at Ocllo sewers 2 times, where the 2 Gargoyls spawns, a bit further in. Sometimes a Stone Harpsie spawn first and then Gamayun but he spawns fast in Cove as far as I have seen. I have been there twice and he spawned every time.
Spent a couple nights farming feathers and my 3rd night he spawned soon after i got there. Covetous lvl 2