What exactly is Korean chili paste, aka 'gochujang'? Is there any way of making it at home?
It is a fermented chili paste made with rice and fermented bean paste.

You could make it at home if you can find all the ingredients. But it looks like a complex recipe and preparation.

For example: https://www.youtube.com/watch?v=5IqtT0HcXUE

Here's a simpler version (less traditional) : https://www.youtube.com/watch?v=QiWJor3DQgI

I would substitute with any other chili paste (sambal oelek...)