The spelling in Sebastian Evans's translation of Geoffrey of Monmouth's Historia Regum Britanniae is "ghittern". Book 9 chapter 1, page 228 of Temple Classics, published in 1904. https://archive.org/details/geoffreyofmonmou00geofuoft
Milton's Use
editWebster's entry on the verb gives Milton as having used it. Neither the Poetical Works nor the Prose Works appear to contain the word. The only place I've found it used in Milton's corpus is in his Poemata Miltoni Manvscripta (on page 40 as numbered in the link, at the end of the second to last line of the first section), reprinted variously (e.g. quoted on page 110 of the introduction of Volume I of Masson's 1874 Poetical Works). I've added this quote to the entry but if someone finds a better one (i.e. one from his published works) feel free to change it.