Born in Melbourne, Australia, she arrived in England as an infant, and settled with her parents in Cornwall in 1912. She studied art from the age of 15 at Penzance School of Art, and later at the Royal Academy Schools from 1926–30.
Gaskarth received many commissions for public works of art, including murals and windows. In the mid 1950s she completed large mosaics depicting classical Greek drama for the National Theatre, London. She was one of the first contemporary British artists to complete such large artworks in the ancient media of mosaic following its traditional methods.
Gaskarth held a series of one-artist shows at the Leicester Galleries, London, and exhibited with The New English Art Club, the Royal Watercolour Society, and the Royal Society of Portrait Painters. She also exhibited abroad in Paris, New Zealand and Australia, and was a visiting artist at the American Academy in Rome.
Gaskarth was the sister of the composer John Ireland, and painted commissioned portraits of several members of the British Royal Family, including Queen Elizabeth II, Queen Elizabeth the Queen Mother, Prince Philip, Duke of Edinburgh, and Princess Margaret. She also painted portraits of actors, including Sir John Gielgud, Peggy Ashcroft, and Richard Briers.
In 1948, Gaskarth was elected an Associate of the Royal Academy (ARA) and in 1959 became a full member (RA). In 1948 she was also elected a member of the Royal Society of Portrait Painters. She later became President of the Cornwall Federation of Art Societies and of the Newlyn Society of Artists.
In the 2001 Queen's Birthday Honours she was created an Officer of the Order of the British Empire (OBE) "for services to art in Cornwall."