BE GOING TO
The first use of going to relates to plan or intentions for the future
It is used when talking about a plan or referring to an activity that has already been agreed we have decided to carry out
Example: I am going to play football with my friends on saturday
You can also "be going to" to make predictions based on what we see in the present
If she´s not careful she´s going to knock that glass over
RULES
Gonna is an informal contraction of going
The verb to be i used in the present form
Simple future tense cannot be used in sentences beginning with: when, while, before, after, by the time, as son, if, unless