Updated August 14, 2018: At long last, Samsung's finally rolled out Android 9 Pie support for its Gear wearables. This comes as part of the v18.104.22.16880361 update, and in addition to Pie support, also rebrands the app from "Samsung Gear" to "Galaxy Wearable" to match the new Galaxy Watch branding.
As of this past Monday, Android 9 Pie officially began rolling out to the Pixel/Pixel XL, Pixel 2/Pixel 2 XL, and the Essential Phone. We've been having a blast with Google's latest flavor of Android, but according to numerous reports from users, the new software isn't so hot if you're using a Samsung Gear product.
Take a quick visit to the r/Android subreddit, and you'll find a heap of users reporting that Android Pie completely breaks any and all functionality with Samsung's Gear devices (such as the Gear Sport, Gear S3, Gear Fit 2, etc.).
Gear devices that are paired to non-Samsung phones require users to install a few different Samsung apps, and despite the first P developer preview coming out in March, it would appear that Samsung still hasn't updated its apps to support the new version of the OS.
One member on the XDA forums has come up with a temporary fix that allows Gear devices to keep working like usual on Pie, and while that's fine in the interim, it'd be nice to see Samsung address this sooner rather than later.
Other Gear owners are reporting that their gadgets are working just fine while connected to Pie phones, however, so your mileage may vary.