It's quirky sounding, but one of my favourite names is
Gwen, just
Gwen, and I don't feel it needs anything more. Thew 'frewi' part can sound really silly IMO, but some people might enjoy that aspect of the name as it is distinctive.
Gwenfrewi is certainly more exciting than
Gweneth (which is the feminine form by the way). I like
Gwendolen as wellbut
Gwenllian is too flowery for me.
Gwenfrewi Melisande isn't the sort of name I'd go for but it flows well.