Translation:A dress is getting torn
Because "gauni" is in the ji-ma class, and in the singular, the verb prefix is li-.
Without "ka", this would mean that the dress is tearing something else, not that it is being torn. I'm not sure if it's passive or what the exact function is, but that is the difference between kupasua and kupasuka. As for li-, it is the prefix for Ji class nouns. Every noun class has its own prefixes.