is used for future meaning when the speaker is talking about plans that have already been made. We usually specify a future time such as next (week, month, year,…), at the weekend, on Monday, etc.
We are visiting my parents on Sunday. (We have already agreed with the parents)
They are buying a new house next week. (We have already agreed with the seller)
Be going to (‘gonna’)
expresses the future plans and intentions.
She is going to get a job this year.
I am going to get up early tomorrow.
Future Simple | Will
We use ‘will’ when there is no plan or arrangement. It is an instant decision at the time of speaking.
We often use ‘I think’ or ‘I don’t think’ before ‘will’.
I think I will have sushi.
I am so thirsty. I will order a glass of cold water.