Are you sure about Gwenlliant? I've only come across Gwenllian. First, the "ll" sound: put your tongue where you would to say "L" and blow. Now the rest of the name: Goo-en-ll-ee-an. Add a "t" if you are sure of the "Gwenlliant".
Gwenfrewi: Goo-en-vreh-wee. The stress comes on the penultimate syllable -- the "vreh". There is, by the way, another version, Gwenffrewi, whic is pronounced Goo-en-freh-wee.