This appears to be the work listed in NGV 1894 and 1905 as a Gill watercolour of “Portland Bay from the Heads” (noted in NGV 1905 as measuring 5½ by 8 inches: the sizes match).
This was the only work of Gill’s listed in the 1905 NGV catalogue (see general remarks on Gill in the linked artist entry).
AR 1872, p.21; NGV 1894, p.113 (V.Buvelot Gallery, 3rd bay, no.21); NGV 1905, p.135 (V.Buvelot Gallery, 3rd bay, no.59) [£1/10]
The SLV catalogue notes that H141719 is “attributed to S.T.Gill by Dr Sasha Grishin” (no acquisition date given)
See also SLV H12921, a pencil drawing of Portland Bay by Gill, dated 1856 and recorded as purchased in 1929