Gwen was born in Southall, educated at various schools in England and felt called to the Church Missionary Society in 1954. She spent many years working in Uganda in religious education and felt that her decades in Africa changed her life and grounded her faith. At that time the CMS in Uganda was headed by the Rev. John V Taylor who later became Bishop on Winchester and a significant theologian. Gwen was forced to leave Uganda during the time Idi Amin was expelling Indian residents whom Gwen was trying to help.
Back in England, Gwen worked with the British Council of Churches.
Cashmore was on the staff of the World Council of Churches (WCC) Sub-Unit on Congregational Life and Renewal in the 1980s. She was pivotal in the worship life and hymnody of the Council, especially in preparing for the WCC 6ht Assembly, in Vancouver, Canada, and well beyond. In 2004, she was appointed by the WCC executive committee to be part of the 2006 WCC Assembly Worship Committee.
Cashmore co-authored the book Clearing the Way: En Route to an Ecumenical Spirituality with Joan Puls. This work explores themes of Christian unity and spiritual practice across denominational lines.
She also contributed to the Dictionary of the Ecumenical Movement, where she discussed the role of spirituality in ecumenism.