An iPhone 6s, SE, 7 or newer is required.
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).
A Cellular model will share the iPhone's phone number, and carriers that support the Apple Watch will charge a monthly fee. In the U.S. the fee is $10 per month added to the iPhone 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