That looks like a lot of gochugaru. I have gochujang but not gochugaru—can I substitute

I would swap in 2TB of the gochujang and i agree with the previous comment to omit the additional sugar and soy sauce.
That will make for a plenty spicy chicken dish as is. If you want it even more spicy you could add a different chili powder or a bit of cayanne but i think i would start with just the gochujang

I think it could work, but I wouldn't add the soy sauce and sugar in the recipe until all the gochujang was added. Since gochujang is already salty and sweet, the soy sauce and sugar might push things over. It seems like the recipe calls for the finer gochugaru powder, not the flakes, so you might consider using another type of chili powder as a substitute instead of the gochujang.