In English, we generally speak in three different times, or tenses.

Three Verb Tenses

the present (right now, the present moment, etc.)
the past (yesterday, last week, before, earlier, etc.); or
the future (tomorrow, next year, next week, etc).

The Future Tenses

There are two verbs that help us talk about the future:

“will”
“going to” (or “gonna”)

Will and going to mean that an action or event occurs at a later time than the speech, whether that’s in five minutes or five years.

Formality

“Going to” and “will” may vary in formality.

Because native English speakers de-stress function words, such as “to”, and connect their speech,

“going to” → “gonna”

which is more informal and casual than “going to” and “will”.

Note: We may say “gonna”, but it should never be written in formal, academic or professional writing. 

Examples

If a speaker says at 2:00pm:

“I will leave soon”

that speaker will absolutely leave soon after 2:00pm, perhaps 2:05pm or 2:15pm.

The same result occurs if the speaker says at 2:00pm:

“I’m going to leave soon.”

He will also leave at 2:05pm or 2:15pm, soon after the time he spoke the words. But the speech sounds a bit more informal.

And the same result occurs with the most informal pronunciation:

“I’m *gonna leave soon.”

Got it?

Let’s practice talking about the future with “will”, “going to” and “gonna”. In these examples, I’ll contract the verb “will” as I would when I speak in everyday life.