The advertising networks are clearly doing business in EU as they're getting paid by these advertisers - so all the major advertising networks will have to be GDPR compliant. So in this regard, the ads (and user info used for these ads) shown to EU users will (in practice) have to be GDPR compliant even if your site doesn't care about it, because everyone who'd want to pay for these ads has to be compliant.
This is the part I'm not sure is true.
Sure, if the ad agency's company motto is "delivering ads to EU users since XXXX", they obviously have to be compliant, but what if they are a US-based ad agency and none of the companies selling them ad space are in the EU? How many layers does one need to circumvent this?
Almost every ad I see is from some company that is actually eager to sell stuff to me, so they're doing business in EU, and have to be compliant.
All advertisers who care about me won't be allowed to buy my data from US-based ad agencies, so even if such US-based agencies can gather the data, it's worthless, since noone would buy that.
I wonder how ads play into all this. E.g. are people who watch an ad on YouTube considered YouTube customers? Are they considered customers of the ad company?
My concern is that we will see more (non-EU) companies implement something where users "pay" for services/features by watching ads. I do also wonder how it then affects the ad companies if they collect personal information about the user. Does the user count as a "customer" if the ad company is the one paying?