Sentence Correction

Aristotle Tense tutorial talks about 2 forms of future tense

will + verb
be + going + to + verb

Which of these is correct on GMAT or both are correct depending on the context?

Both forms are correct. Their usage depends on the context.

Future Simple is used:
- in Conditional 1
- when we are giving information about the future, or predicting future events which are not already decided or obviously on the way
- to express intentions and attitudes towards other people

Be going to + verb is used:
- to talk about plans, but usually in an INFORMAL STYLE
- to predict the future on the basis of present evidence - to say that a future action or event is on the way or starting to happen