I think the name would be pronounced as /ɡwɛnˈwənwɨ̞n/ or /ɡwɛnˈwənwɨ̯n/ in the North of Wales, and /ɡweːnˈʊi̯nʊi̯n/, /ɡwɛnˈʊi̯nʊi̯n/, or /ɡwɛnˈwɪnwɪn/ in the South, but given that it's such an unusual name, I don't think I can verify this at all easily.Edit-- i messed up the stress; this has been remedied.
Edit-- i messed up the stress; this has been remedied.