Chan eil dona is in several text books and features on BBC.
Do people ever shorten it? From say «chan eil dona» to "na dona" (guessing). I keeping thinking I hear shorter responses ("cha dona" ??!).
Chan eil gu dona...is that not...not to bad instead of not bad?
No, gu just converts adjectives to adverbs.
"Tha mi gu math" means
"I am (doing) well."
So "chan eil gu dona" would be "(I'm) not doing badly."