I'm thinking about this translation and how the word "really" can have two subtly different meanings in English. "Really" could be a virtual synonym for "very", as in describing the quality or extent of the darkness (pitch black), or it could be introducing uncertainty as to whether or not darkness is actually present. For example if someone has been unaware of the time and is informed that darkness has fallen, they might query this with mild incredulity as "Is it really dark?" That is, are you telling me the truth about its being dark? I'm presuming that the former is the sense meant with gu math, but would "very" not be a less ambiguous translation?
Hah! I get that if you were expressing doubt about whether or not it was actually dark in Gaelic you wouldn't say "A bheil i gu math dorcha?" I was just musing that an English speaker might be expressing some incredulity that it was actually dark when they used the phrase "Is it really dark?" And hence, that "Is it really dark?" might not be the most unambiguous translation. But then there's probably a reason why you aren't translating "gu math" as "very" (which would be my choice for an unambiguous phrase in English) and maybe I'll find out why not at some stage.
I suspect that the course designers may have settled on 'really' for 'gu math' to differentiate it from the 'very' of 'glé', although they do seem to be largely interchangeable. I suppose the software can only cope with so much subtlety and choice. I rather like 'gu math', whose closest literal translation would be 'well', as it reminds me of the English teenage slang use of 'well' : 'He's well minted' or even 'That was well good': 'gu math math!' Ain't language fun!