These two are very mixed up in usage – even if the basic rule is very clear:
“God” is adjective
“Bra” is adverb
So you would say “en god bok” for a good book, and “han snakker bra” for “he speaks well”.
In lots of cases one could use them interchangably, but the safest route is to stick to the rule, and then just learn one exception/expression at a time.