This will tell you want you can do with a GPS only when your iPhone is nearby and when it is not
https://support.apple.com/en-us/HT205547
Make sure you read the footnotes.
An Cellular model where you do not activate the cellular will work just like a GPS only model. Since you can only get the stainless steel model with cellular, users that do not use one of the supported carriers or do not have a carrier that supports the Apple Watch in their region, or just do not want to pay the monthly fee for cellular, can use the stainless steel model as if it was a GPS only model.