Definition of "gallnut"

There was the smell of evening in the air, the smell of the fields in the country where they digged up turnips to peel them and eat them when they went out for a walk to Major Barton's, the smell there was in the little wood beyond the pavilion where the gallnuts were.Macmillan Press Ltd, paperback, page 62

1916 December 29, James Joyce, A Portrait of the Artist as a Young Man, New York, N.Y.: B[enjamin] W. Huebsch

[Y]our hands are always covered with gallnut [translating galla], vitriol, and alum, just like the varnish on a Moor.

2007, Giambattista Basile, translated by Nancy L. Canepa, Tale of Tales, Penguin, page 195