"Ciamar a tha thu? Chan eil gu dona."
Translation:How are you? Not bad.
Excellent, if you are trying to produce good English. But they are trying to teach Gaelic here so they will always prefer a more literal translation, or people will be saying 'Which Gaelic word corresponds to too?
Chan eil gu dona 'Not bad'
Chan eil ro dhona 'Not too bad'
Note that you do not use the gu in Gaelic when you add another adverb, such as ro. D
There have been discussions on this page about about adjectives and adverbs - gu here is the adverbial marker -ly in English, but no one suggesting that it means 'really'. There may be two sources of confusion. Firstly, someone else has suggested 'not too bad' and you could have said 'not bad, really'. Here the too and the really are not acting as intensifiers, but actually reducing the force of the bad.
The second thing is that gu math 'well', rather than just gu by itself, is used as an intensifier. Thus
Gu math dona 'well bad' - 'very bad'.
Hopelessly wrong Duolingo answer. "gu dona" doesn't mean "bad" ("bad" is not an adverb). It means "ill" or "badly" or "not well". So the official answer is wrong, and so is the hover hint that tries to make "bad" an adverb ("gu dona" is an adverb - "gu" plus an adjective is always either an adverb or an error).
Well I've only been able to speak Gaelic for about 20 years, so if you have more experience I will bow to your superior knowledge, but I think this translation is fine.
In English we are quite happy to say "I am well", using the word well, the adverb from good as if it is an adjective in this context. Well in Gaelic, as far as I know (and everyone else I have met who speaks Gaelic), we do the same with gu dona, the opposite of gu math 'well'. Why we use adverbs as adjectives here is not obvious. I guess that (in whatever language) there is some verb missing: "I am well" is actually short for something like "I am doing well" or "I am going well". After all this is what you say in French, for example
Je vais bien (adv)
I am (going) well (adv)
The difference between French on the one hand, and Gaelic and English on the other, is that the French simple verb vais cannot be abbreviated, whereas the compound verbs am going and tha ... a' dol can, just by not bothering with the last part.