If I buy an Apple Watch with cellular do I automatically start paying for it or can I use gps for the time being?

If you buy an Apple Watch with a cellular radio, then you pay more for that specific model, because it has extra cellular radio hardware and the built-in eSIM inside. You may also be paying more for a different more expensive case, or a different more expensive watch band, or because you got the Nike Branded model. This is a one-time upfront cost.

YOU DO NOT pay anything extra to a cellular carrier if you have an un-activated cellular radio. You ONLY pay a cellular carrier if you activate the cellular radio, and that is generally a monthly fee as long as you keep the cellular radio activated (In the U.S that is around $10 per month).

If you buy a GPS only Apple Watch, then you do not pay extra for an unused cellular radio. But you cannot add cellular to that specific Apple Watch in the future either.

I have been using a GPS only Series 4 for the past 3 years, and I have been happy with it, as my iPhone is almost always with me, so I get all the functionality, with out the extra cost.

My Wife has a Cellular Apple Watch, because she does not want to carry her iPhone with her all the time.

You may use a cellular model in GPS only mode, and activate the cellular at a later date if you so desire.

To activate the cellular radio, you have to contact your iPhone cellular provider and have it added to your iPhone plan.

The GPS only Apple Watch can do Everything the Cellular model can do AS LONG AS your iPhone is nearby, where nearby is generally somewhere in the same moderately sized home. The Cellular model allows you to leave your iPhone at home while you go for a walk, run, bike ride.

The Cellular + GPS Apple Watch has an eSIM and shares the iPhone's phone number, and the carriers that support the Apple Watch charge a monthly fee. In the U.S. the fee is $10 per month. Here is a list of carriers that support the Apple Watch:

https://www.apple.com/watch/cellular