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Maybe GOG might have thought about starting a big sale already. The problem: The Outer Worlds would have had a release during that sale. And might not get the attention GOG/Obsidian Entertainment wanted. (Who knows since when the agreement exists, releasing TOW on this date on GOG. So it is just an assumption within long terms.)
Releasing a game before a sale sounds like making more sense, than releasing it during a sale. Because during large sales the rivalry between products is increased by customers buying the much cheaper games, than the new titles. (Even though TOW starts with a nice 50% discount.)
That would just be my idea why GOG has not started their Halloween/Fall Sale yet, besides starting a large sale 1 or 2 weeks before a steam sale makes a lot more sense.