If you buy the cellular model you don’t need to set up and activate cellular on it until later on. Once you do, it’s an add on plan about $10 a month extra with the carrier you’re using for your iPhone. You would need to ask your carrier how much it costs. Activation can be done by you on the watch app. No real need to call the carrier and the price should be displayed in the app via the carrier site during set up before you continue with “Finish”
GPS can still make calls, texts but needs to be nearby your iPhone to work. It would use wifi calling which needs to be supported by the carrier.
Set up cellular on Apple Watch - Apple Support