For native speakers, there is little difference between will + verb and be going to + verb. In most cases, they are used interchangeably. But if you are taking an English language exam, they will insist on distinguishing nuances of meaning between the two forms, so it's good to know the rules. This website is one that I frequently use to teach grammar. https://www.englishpage.com/verbpage/simplefuture.html It says you can use either form for predictions. USE 4 "Will" or "Be Going to" to Express a Prediction Both "will" and "be going to" can express the idea of a general prediction about the future. Predictions are guesses about what might happen in the future. In "prediction" sentences, the subject usually has little control over the future and therefore USES 1-3 do not apply. In the following examples, there is no difference in meaning. Examples: The year 2222 will be a very interesting year. The year 2222 is going to be a very interesting year. John Smith will be the next President. John Smith is going to be the next President. The movie "Zenith" will win several Academy Awards. The movie "Zenith" is going to win several Academy Awards. IMPORTANT In the simple future, it is not always clear which USE the speaker has in mind. Often, there is more than one way to interpret a sentence's meaning.

I was actually going to point out that the rule isn’t 100% strict, but the truth is that your example works much better as is — future with “will”. The reason is that the meaning is conditional, “if you go out (against my advice), then you will get wet.” The situation now is that I’ve advised you not to go out, so there’s no reason for me to expect you to get wet. If, on the other hand, I expected you not to follow my advice, then I would say, “well, then you’re going to get wet.”

I'm not sure about the grammar book. In my experience as an english speaker, 'You're going to get wet." would be correct but so would 'You'll get wet'. To me both speak to the expectation that you'll get wet if you go outside. In this example any difference in meaning is very subtle. But I do not teach English grammar. I did a little research and you might like this page: http://www.grammar.cl/Notes/Future_Will_vs_Going.htm As you can see, either can be used for predictions. I'd always use "going to ..." for plans. I'm going to the movies. He's going to cook spaghetti for dinner. Generally, use "will" when you are offering to do something or asking for someone to do something.. "If you are cold, I will close the window." "Will you help me with my English." The page gives other specifics. Good luck.

Yea you're right.. It's not wrong to say you will get wet or you're gonna get wet. Both are right. The first has more emphasis on 'will'. It's just a more formal way of saying the same thing.