Gillies divided his energy between oil and watercolour; painting in oils was his studio routine, whilst he was happiest working in front of the landscape on his watercolour block. The originality and spontaneity of his landscapes on paper in pencil, pen and ink and pure watercolour, form the core of his reputation. When filmed in 1970, he spoke eloquently about his watercolour practice:
My landscape painting began with watercolour and a great part of my work has continued in this medium and I feel the peculiar qualities of the medium have had a strong influence on my conception of landscape… I have perhaps opened many people’s eyes to some unexpected, some subtle beauties in our daily surroundings. This has been, I hope, a by-product of my own enjoyment of what I perceive and my great delight in the very act of handling the paint.