I'd say
Math like thistle, or like
Beth - ditto
Gethin.
Corroboration:
http://www.forvo.com/word/gethin/#cydd is usually more like 'that':
http://www.forvo.com/word/gwynedd/#cyhttp://www.forvo.com/word/mawddach/#cy
I have no idea how much pronunciation has changed since the 18th century, but probably not a lot :)