Gwin
It rhymes with "win", "spin", and "thin".
There are some alternative pronunciations, depending on the context and origin of the name.
* Gween (rhymes with "queen") - This is more common in Welsh-speaking areas.
* Gwayne (rhymes with "Wayne") - This is less common, but sometimes used in English.
It's always good to ask the person how they pronounce their name if you're unsure.