Cellular+GPS vs GPS

I have a GPS only Apple Watch (as well as my Wife) and we exchange messages all the time. We also answer phone calls on our Apple Watches (we could make them too, but we rarely actually call anyone). We also use the Walkie-Talkie feature to talk to each other as well.

SMS/MMS texting does require that you iPhone be turned on, and connected to the Internet, even if you are using a cellular model. The cellular model just doesn't need the iPhone nearby.

This describes what you can do with both models when the iPhone is not nearby

https://support.apple.com/en-us/HT205547

It is strongly suggested you read the footnotes as well.

And by "Nearby", typically that would be your iPhone and Apple Watch are somewhere in the same 3-4 bedroom home. My Wife frequently has her iPhone in the kitchen charging, but can use her Apple Watch in the diagonally opposite side of the house on the 2nd floor.

Also if your cellular carrier supports WiFi calling, and you enable it on your iPhone and Apple Watch, then the Apple Watch can use WiFi calling when it is attached to a WiFi network.

No you can make calls and send texts with a GPS only watch,the only proviso is that you have to have your phone with you.