
Gillian studied at The Royal Academy of Music and appeared in concerts, oratoria and on television with the Linden Singers whilst still at the RAM. She joined the D'Oyly Carte Opera Company in 1959 as principal contralto and appeared in all the principal contralto roles: Little Buttercup, Ruth in The Pirates of Penzance, Lady Jane, the Queen of the Fairies, Lady Blanche, Katisha, Dame Hannah, Dame Carruthers and the Duchess of Plaza-Toro during her six years with the Company.
She can be heard on recordings of H.M.S. Pinafore (1960), Patience (1961), Iolanthe (1960), Ruddigore (1962), The Yeomen of the Guard (1964) and The Gondoliers (1961). In 1960 she married Trevor Morrison who was the D'Oyly Carte master carpenter and left before the birth of daughter, Rebecca.
She returned to the (New) D'Oyly Carte in 1988, their first season, to play the Queen of the Fairies and Dame Carruthers. In the meantime, she frequently returned to Gilbert and Sullivan including the 1966 BBC radio broadcast of The Pirates of Penzance and voiced Dame Hannah in the 1967 Halas and Batchelor cartoon of Ruddigore as well as appearing as Lady Jane in the Sadler's Wells Opera production of Patience in 1969-70.
She also appeared with Thomas Round and Donald Adams in Gilbert and Sullivan for All and toured North America with them.
In 1970 Gillian made her Covent Garden grand opera debut with the title role in Bizet's Carmen which brought her international acclaim. She played several performances opposite Placido Domingo. She went on to perform in Rigoletto, Il Trovatore, Otello, Falstaff, Die Meistersinger, Semele, The Fiery Angel, Salome, and Wagner's "Ring" cycle. She has sung at Paris Opera, at the Tanglewood Festival, in Pittsburgh, in Frankfurt, in Basel and with Scottish Opera, Opera North, and the New Israeli Opera.
Further appearances include roles in the Brent Walker television series and on BBC Radio 2 in 1989 when she sang seven contralto roles adding Lady Sophy and the Baroness von Krakenfeldt to her repertoire. In the 1990s she joined the newly reformed Carl Rosa opera Company and toured in several Gilbert and Sullivan operas recording Katisha in their 2001 video of The Mikado.
Also in the 1990s she appeared at the Buxton International Festival both on the stage and to run master classes and be interviewed about her life with Gilbert and Sullivan. During this time, she appeared on a recording of The Martyr of Antioch created at the 2000 Symposium and made her final performance in Carl Rosa's The Yeomen of the Guard at the Festival.