"I like swimming" is strictly speaking ambiguous; it could be a verb (cf. "I like to swim") or a noun as in "I like occasional swimming".
Yes, gerund-participials can be clauses, sometimes catenative, and sometimes not.
Most students in the early stages of their studies are not taught catenatives, so it's always difficult to advise without contradicting their teachers. I prefer the catenative analysis where it is applicable, but as I said "I like swimming" is ambiguous, and sometimes it is better to teach it as noun to beginners.