The pre-Cromwellian gurn

As fine an example of the gurn in English literature pre-1600 as I have ever come across:

XIV
Who came at length, with proud presumpteous gate,
Into the field, as if he fearelesse were,
All armed in a cote of yron plate,
Of great defence to ward the deadly feare,
And on his head a steele cap he did weare
Of colour rustie browne, but sure and strong;
And in his hand an huge polaxe did beare,
Whose steale was yron studded, but not long,
With which he wont to fight, to justifie his wrong.

XV
 Of stature huge and hideous he was,
Like to a giant for his monstrous hight,
And did in strength most sorts of men surpas,
Ne ever any found his match in might;
Thereto he had great skill in single fight:
His face was ugly and his countenance sterne,
That could have frayd one with the very sight,
AND GAPED LIKE A GULF WHEN HE DID GERNE,
That whether man or monster one could scarse discerne.

The glossary in my edition of The Faerie Queene says this means 'grin', but really. I mean, really. And that's all I have to say about that matter.