From the Tips & Notes:
"Zullen + infinitive is used in the following cases:
when saying something will likely take place when emphasizing that something will definitely happen to promise or propose something
Gaan + infinitive is used as follows:
when expressing an intended action when something is going to take place, without expressing probability"
I keep,trying to equate ga with 'going to' (since gaan is to go) and zal with "will" (since I think it's cognate with "shall")...but it always seems to be the opposite of what I expect.
Still, it just seems like shades of meaning, as opposed to drastically changing the whole sentence.