Aaaahhhh, it doesn't accept kanji for gymnastics... reported
It registers as wrong if you don't include です, but it should be correct.
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.
Could you use 見ることが好きです instead? Or are こと and の interchangeable?
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).
Should accept は as well.
Must you use が? Is using は wrong?
It is not a must and は can be used if you want to topicalize the phrase 体操を見るのが. は supesedes が and を so in this case it becomes 体操を見るのは
体操を見ることが好きです。Is not accepted, but google translates it no problem.
Sad this is my very last question
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.
I feel like the "no" should not be here?
It is required for nominalisation (Verb + 'No' = Noun).
It was not required in ancient Japanese but required in modern Japanese. For some specific idioms の is not needed, like 見るや いなや(as soon as I see)，見るに たえない(cannot bear to look)