Apple Watch
So if I buy an Apple Watch (GPS+Cellular) do I have to buy a cellular plan to use it, or would it function just like the GPS model until I bought the cellular plan?
Apple Watch
You are not required to enable and pay for cellular service. In that mode, it will behave just like a GPS only model.
What the Apple Watch GPS-only can do without an iPhone (there is information about the cellular model as well)
https://support.apple.com/en-us/HT205547
https://www.imore.com/heres-what-apple-watch-can-do-without-iphone
GPS vs Cellular
https://www.imore.com/apple-watch-cellular-vs-gps-whats-difference