This is what I was looking for, from the BJP 1896, so the earliest mention of this with a "recipe":
On an Improved Method Of Coating Paper for Direct Pigment Printing by Maskell
Maskell is excited because Demachy has discovered a new method of gum printing that deserves all the merits of a new discovery. Although Rouille Ladeveze is responsible for reintroducing the direct coating of paper with a bichromated compound, his directions said the coating should be neutral and not of an acid character. Demachy, however, has discovered that the principal factor in the successful coating of the paper was that the mixture should be a distinctly acid one. Take equal quantities of a very thick gum mix and lemon juice, and mix to a glutinous, soft syrup. Add the bi and pigment to the thickness you need is obtained. If the whites "clog", add a few drops of essence of thyme (the essential oil). "What the action of the acid may be must be left to scientific men to determine, if they deign to notice the subject. It may be humbly suggested that it may be favorable to the production of chromic acid.."
Maskell calls this the acid-gum process
Christina Z. Anderson