Can you turn GPS off on a Apple Watch Series 2?
According to Apple, the built-in GPS of the Apple Watch series 2 will not kick in if the iPhone the watch is paired to is within range. 

So, depending on your workout, having the iPhone with you (or nearby) will result in the iPhone doing the grunt work to conserve power usage of the Apple Watch.

Also, even if your iPhone isn't nearby, the Apple Watch GPS will not kick in unless you're actually doing a workout. For example, if you forgot your iPhone at home when you went to work and did nothing but sit in a car or at your desk the whole day, then the GPS would have never kicked in.

As for a setting to manually switch the GPS off, there wasn't one in the beta versions of WatchOS 3, but there are rumours that a future update may add this option.

Hope this helps.

[EDIT]

I just wanted to clarify that whether the Apple Watch GPS kicks in or not will also depend on the actual workout. Basically, if it's an indoor workout it won't, if it's an outdoor workout it will (i.e. if your iPhone isn't nearby). However, if it's a swimming workout the GPS only kicks in if it's an open water swim (i.e not in the pool) and you're doing freestyle. Any other stroke the GPS won't kick in.