Whew! I have been trying to research "Magnolia gigantea", and I haven't had much luck finding it. If you are referring to Magnolia acuminata 'Gigantea' (which is apparently quite rare) it should flower shortly after the leaves expand, in late spring and early summer, and apparently should be pruned right after that. I suspect that pruning it in late summer or fall will be hard on next year's bloom, at least. If you join GoY, which is free, you could describe it to us, and we might be able to tell if it's a different species, or if you just got a bonus this year!