It can also mean 'They are naked in the evening' depending on the context ;)
"I am empty in the evening" As well, yes?
Also, I am a native english speaker and I have no clue what is actually being said here... Does this mean "I am free this evening" Or does this mean "My stomach is empty this evening" Or "I have no energy this evening?"
'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.