I imagine outright banning would create a fairly large grey market. The objective should be harm reduction, as eradication would be basically impossible.

The discussion is about banning the advertising of gambling, not gambling itself.

There won't be a large grey market for advertisements.

Wanna bet?

Try regulating that on the internet, or walk down a construction sight in Manhattan, there are illegal ads all over.

Easily solved.

If someone shows the regulator an ad for fanduel that shouldn't exist, they pull their permit to operate.

We have already seen that you can ban ads pretty effectively. I can't remember the last time I saw a cigarette ad, hell, where I live you can't even display them openly in stores, I can't even recall the last time I saw a cigarette logo.

I have yet to see any 'grey market' cigarette ads.

I think we just disagree about the definition of "large" in this context.

The market for what you mention is less than 1% the legal market.

There is big difference in signaling of social acceptability with advertising existing e.g. as main sponsor of the superbowl vs. existing as Stake logos on social media videos.