babyjn you can use the watch yes, it will pair with any iPhone. You cannot however use any of the cellular features of the cellular models. So if you buy just a gps version you’ll get all the features available on giffgaff. If you buy gps + cellular the watch will still work, but the cellular part won’t.. so you wouldn’t (for example) be able to go on a jog without your phone and still get text messages on the watch.
IMO the cellular version is a waste of money anyway. Those features (calling and texting from the watch) aren’t very useful.
the god model I have works great with giffgaff