de0u You can "Transfer Phones." But it shows "Transfer failed" whenever GrapheneOS tries to pair right after the message "Continue on phone" appears. You cannot pair via bluetooth to the google watch 2, even though bluetooth has to be enabled, you can only pair via the dedicated pairing system. If your watch is already paired with another phone, the bluetooth pair screen on the watch and phone will not see each other, but will if you go to "Transfer watch" in system or the notification that appears when the pixel watch app is trying to search.
If you manage to get past this issue, the google watch app will show a dialogue that will ask for a pin number. The dialogue will VERY QUICKLY close itself, and the app will immediately re-request pairing.
After this, the watch itself will go into a transfer failed loop and the app will keep re-requesting a bluetooth pair code. This will happen even if you give both play services and pixel watch app all permissions. Opening pixel watch app will complain that play services doesn't have nearby devices permission, even if it does.