To be very clear. You Must have an iPhone 6s, SE, 7 or newer to use an Apple Watch. The Apple Watch is an iPhone accessory.
The cellular models share the iPhone's phone number. iPhone carriers that support the Apple Watch will charge a monthly fee to share the iPhone phone number. In the U.S. the fee is $10 per month added to the iPhone monthly bill.
iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:
https://www.apple.com/watch/cellular
The Apple Watch GPS only can do Everything the cellular model can do as long as your iPhone is nearby, where nearby is somewhere in the same 3-4 bedroom home (assuming typical wood frame construction).