It can be either present perfect "she has gone for the day" or simple present "she is gone for the day" depending on context: a. I'm looking for Linda. When will she be back?? Sorry, she has gone for the day b. I'm looking for Linda. Is she here? Sorry, she is not coming in today. What! She is gone for the day?

Has! Present perfect! Kate has (Kate's) gone for the day - I have (I've) answered your question. You use present perfect because the event has already happened but it is still true now. Kate left earlier, but her absence is still relevant to us discussing it now. I have already answered your question and it is still true / answered now. It would also be correct to just use the past tense: 'Kate went earlier', 'I answered your question' - there's not a big difference between the two. When words are contracted (shortened) like 'Kate's' or 'it's', we only do this for words in the present tense, so 'it's' can only mean 'it is', never 'it was' - you need to say the word 'was' when speaking about the past. We contract words like this when were speaking - you should try to do this because it sounds more natural. When writing, if it's formal writing, you should write the full words and not the contractions. With informal writing, like on italki, or when writing dialogue, it's ok to write the contractions.