In Grammar and Vocabulary for First Certificate (Prodromou, 2005), Chapter 2 covers "the future", "be going to", "present continuous", and "present simple". I put three of the End-Of-Chapter questions below with the corresponding back-of-the-book answers. My concern is with Question #1 only. (I wrote #2 and #3 for context.)
Put the verb in brackets into the most suitable form of the future.
"What ________ you __________ this evening?" (do) 'Nothing.'
[Answer: are ... doing]
"Well, _______ we ___________ to that new pizzeria?" (go)
[Answer: shall ... go]
In 2004 the Olympic Games _________ in Athens. (take place)
[Answer: will take place]
For Question #1, the only answer in the back-of-the-book answer is the present continuous: "What are you doing this evening?" This has effectively ruled-out the possibility of using 'be going to' to complete the sentence. In other words, I think what he is saying is that "What are you going to do this evening?" is incorrect.
Do you think he is right?