Gwilym was hired by S4C, a Welsh-language television channel, in 2004 as the editor for factual programmes and co-productions. Her productions for S4C included O'r Galon (From the Heart), Wynebau Newydd (New Faces), Cefn Gwlad (Countryside), Ffermio (Farming), and nature programmes hosted by Iolo Williams. She was a content editor on the current-affairs programmes Y Byd ar Bedwar (The World on Four) and Wedi 7 (After 7). In 2010, she won a BAFTA Cymru award for her on Dwy Wraig Lloyd George (The Two Wives of Lloyd George).
