No luck for an original version of that Diccionario Castellano of 1787. I could swear that I saw this word in some pieces of Argentinean literature of 1810 to 1830, referring to Germans, but can't remember a title. We'll see. Gabacho is of Spanish origin too, from the region of Catalunya, derived from an ancient uncertain term GAVATXO which meant French.

Un abrazo - Andrés