Can I use an Apple Watch series 6 without using cellular?

There is no GPS-only version in stainless steel, so buy the GPS+Cellular version and don’t provision the cellular on the device. It’ll then function entirely as a GPS version and will leach off your iPhone while that’s in range, until and unless you later decide to add it to your iPhone cellular plan.

Yes you can use Watch without the cellular part.

All you need is that your paired iPhone is nearby to take phone calls on your Watch.

The question is why are you buying Watch GPS + Cellular? It's a bit more expensive as well.

You can normally buy Watch GPS without cellular (of course depending on the models)

You can purchase a cellular version and choose not to activate with your carrier.

I have done this; in Canada.

Then yes, you are correct.

The stainless steel model do not have GPS only option.

Anyway, you can buy Watch GPS + Cellular and use like a GPS only Watch.