Timboli: Just for the record, after I grab a free game from GOG that changes my subscription settings, I immediately change them back, and I've never received any related marketing from trusted GOG partners.
Take that how you will.
So maybe it isn't an instant thing, perhaps email addresses for those who have subscribed get collected at the end of the day or week or month, and then sent off to partners. Makes more sense that it would be that way, to reduce system resources etc.
I haven't unchecked anything in like 3 years at least, and I'm also not getting any spam from outside GOG.
From what I remember reading in some FAQ explanation, this "trusted partners" thingy is widely misunderstood, but I won't find it now. But it didn't mean that GOG is sharing your e-mail with anyone.