From Professor Doctor P.C.H. Schrijver, Department of Languages, Literature and Communication, Celtic, Institute for Cultural Inquiry, University of Utrecht -
"Linguistically, the first thing that comes to mind regarding the initial alternation Usai /Iusay is the pair OW iud, MW udd 'lord' < *iüdd. So OW word-initial j- disappears in front of ü (= MW u). As to your assumption that Iusay may be connected to Gewissae if there is a rule that states that medial -i- is lost, I can tell you that there is indeed such a rule: *wi > ü in non-final syllables (as in *wikanti: > MW ugeint, see my Studies in British Celtic Historical Phonology 159-60). This generates the ü that we need in order to later get rid of the initial j. The only remaining problem is connecting OE Ge- /je/ with OW j-. Barring that, I would say, yes, what you suggest is possible. That still leaves the origin and etymology of the name in the dark (the reconstruction leads to something like *iwissai- or *g/jewissai-), but first things first."
From Professor Doctor Stefan Zimmer, Department of Celtic, University of Bonn -
"Spontaneaously, your idea of interpreting "Iusay" as a W form of OE Gewisse seems quite attractive. One must, of course, check meticulously the palaeographic details. As I am, alas, not a palaeograher myself, I cannot say more. I see no "LINGUISTIC" problems."
From Professor Patrick Sims-Williams, Department of Welsh and Celtic Studies, The University of Wales, Aberystwyth -
"I suppose Ius- is the older form and became Us- like Iustic in Culhwch which becomes Usic. Forms of Gewissae are noted by Williams/Bromwich Armes Prydein pp. xv-xvi. One Welsh form is Iwys, which rhymes as I-wys, and as the diphthong wy can become w, you could get I-ws- which could be written Ius- in Old Welsh and then add -ae from Latin which almost gets you to Iusay."