In formal English (or written English), I'm super sure that the Be going to and Will rules are "respected". (Correct me if I'm wrong) For example, probably in formal English, they probably use "Going to" for things planned and "Will" for things decided at the moment of speaking.
But, does this happen in Colloquial/Iformal English? Sure, we've got the expressions "I will help ya!" (Instead of "I'm gonna help ya"), and "I'm gonna visit my grandma next weekend" (instead of "I'll visit my grandma next weekend), but, sometimes I see "going to" instead of "will" and when someone use "going to" or "Will", I wonder why, because, the rules say that "going to is for that" and "will is for this", though, I don't know why someone use this tense instead of that one.
And, Of course there are native speakers that don't know the difference between them. The thing I wanna point out is: Is there really a difference between "Going to" and "Will" in Informal English? Or, the rules are just for formal English only?