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I'd say Math like thistle, or like Beth - ditto Gethin.
Corroboration: http://www.forvo.com/word/gethin/#cy
dd is usually more like 'that':
http://www.forvo.com/word/gwynedd/#cy
http://www.forvo.com/word/mawddach/#cy

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I have no idea how much pronunciation has changed since the 18th century, but probably not a lot :)