https://www.cnet.com/news/the-galaxy-note-9-works-with-gear-vr-via-free-adapter/

The Samsung Galaxy Note 9 is slightly larger than the Note 8, so it's an uncomfortable fit into the company's Gear VR headset. And you don't want your new $1,000 phone to be uncomfortable. So Samsung will provide you with a free adapter (which we knew was coming) -- but you've got to call, like in the olden days.

At least, that's what the company tells you to do on the support page. Some folks have had luck texting through the Samsung Plus app, though.