Yes – if you cannot find gochugaru, you could substitute other chili flakes (or make your own). Gochugaru is a fairly mild (between 4k-8k Scoville units) with a slightly sweet, smoky flavor. In comparison, US/UK "red pepper flakes" (often made with cayenne) are more like 30k-50k Scoville units – so while the sub is convenient, the heat will definitely vary, and the amount will need to be reduced. Aleppo or Kashmiri chilies are potentially the best substitutes. Chile de Arbol may also work well (15k-30k Scoville units).