"I like watching gymnastics."
Put a だ at the end to form a complete sentence. (好きだ。) I think as a good translator we should aim for translating to Japanese complete sentences if the English sentence is complete, and vice versa.
They are interchangeable in this case because the の means こと (action). But in sone other cases they are not interchangeable. e.g. 彼が走っているのを見ました where the の means さま(state). 残（のこ）っているのをください where の means もの(object).
Sorry, guys. The pronunciation of Taiso puts stress on 'I' therefore ta I so. The audio sounds like putting stress on Ta which sounds very odd.