That's just my take on your questions above.
There are many things that aren't taught about 'be going to & will' in our curriculum. One of these things is that both can be used for prediction almost interchangeably. That's why most native speakers see that both could work in most of our fabricated exam questions.
- I see that 'will' works well in your first two examples and in Sara' examples above. A simple question could help you to choose easily here: Are you talking about 'certainty' or 'proximity' because of present reality or evidence? If you are talking about certainty, use 'will'. If you are talking about 'proximity', use 'be going to'.
- Ali is running fast. He is going to win.
- He is dead tired. He is going to sleep.
(It is near. 'be going to' is related to the present).
- Ali runs fast. He will win.
- He is careless. He will never achieve his ambitions.
(It's my guessing based on my knowledge. 'Will' is always related to the future).
Sometimes both could work and be used interchangeably.