G1/2B to NPT Coupling
The G in the thread size looks like it denotes a British Standard Pipe (also known as BSP or BSPP) size of 1/2".  For whatever reason, Kholer decided to use this size on their hand showers, and that's what this valve is designed for.

You would need to find an adapter such as this one to convert to NPT. This listing mentions shower fixtures, so that is promising.  The title and description contradict each other on which side is BPT and which is NPT, but the product images look like it's what you need (they show female BPT threads).

Here is another possible adapter, but not chrome plated.  If you are in the UK, you may be able to find a BSP to NPT adapter easier.