Still have problems : what's the difference between "I'm good" and "I'm well" - the first one meaning "It's OK by me" ("I don't need anymore - you can have it") or does it mean "I'm a GOOD person" (a NICE person) ... and the second one meaning "I am fine / I am well / I'm OK" - do I unstand this (English) problem correctly ... "tha mi duilich - tha mi à Bavaria" ;)
I am good meaning 'It's OK by me' is a very modern colloquialism in English that has not (as far as I know) ever been used in Gaelic.
I think the important thing is not to worry too much about what math would mean as it doesn't say that. As 1e7nx0WG suggests, the translation as good is simply wrong.
Gu makes an adjective into an adverb, so gu math means 'well'. Why we say
Tha mi gu math
I am well
in both languages is not obvious. We don't say I am badly or I am finely. Actually in Gaelic you do say
Chan eil mi gu dona
I am not bad
so in fact the Gaelic is is some ways more consistent.