'Goi' is a plural adjective so in this sentence we'd have to translate it as 'They are' (remember that 'sunt' can be both 'I am' and 'They are'). If you wanted to say 'I am empty in the evening' (which would be either a nonsensical or very existentially depressed sentence) you would have to say 'Sunt gol/goală seara'. 'Sunt goi seara' could be talking about a restaurant or a cafe who don't have any customers in the evening - hence 'They are empty in the evening'. That's the number one problem with Duolingo's model - you need to learn language in context.
I find this to be a bit of an odd sentence.. it's not necessarily very easy to understand what is meant to be said here. Perhaps it would be wise to add a few words to this to make it easier to comprehend? Such as "The store is empty in the evening." or something along those lines. :)
I have heard the sentence (in a folk song) copiii sunt goi, meaning that the children have empty stomachs/not enough to eat.
Context: Where are there nudist colonies in Romania? Do they dress for work?