If I buy refurbished apple watch series 7 without GPS Celular will it synch with my iPhone se 2020 and use its cellular and GPS

All Apple Watches include GPS. There is no model without GPS.

The Watches without cellular will use the iPhone's cellular connection while near by, or can use Wi-Fi when away from the iPhone to make calls and get messages.

See here for more details: link-> Use your Apple Watch without your iPhone nearby - Apple Support

And yes, it will pair with an iPhone SE 2020.

iPhone SE2020 will accept iOS16 so any Apple Watch 7 with iOS9 will pair to it.

I always worry about "Refurbished" watches ad iphones.

If Apple refurbished it then no problem.

If the seller cleaned it up and maybe put on a ew strap, no problems.

If the seller took it apart then anything could happen.

LD150 wrote:

I always worry about "Refurbished" watches ad iphones.

But it can be OK if it's a reputable dealer who gives a trustworthy guarantee (like Apple does)

Reputable as in Apple Approved technician who maybe replaced the battery. I would not trust an "Amazon Renewed" for example - meaningless.

Perfect