https://www.cnet.com/news/samsung-galaxy-s9-gear-vr-compatibility-note-8-daydream-view-mwc/

The appeal of phone VR is simplicity and affordability: Just use a $100-ish set of goggles and your phone, and you're set. Samsung gummed that up a bit over the last couple of years by requiring slightly different new models of Gear VR to fit its newer phones, most recently the Note 8 last year.

Samsung's newest phones, the Galaxy S9 and S9 Plus, didn't debut with any new VR hardware. But they do work with the Gear VR. However, they only work with the Note 8 model released in September last year, not the nearly identical Galaxy S8 version that debuted in March that same year.