Quite a few years ago now Dr. Richard Coates clarified the etymological origin of the tribal name Gewissei or Gewissae. He linked the name, correctly, to Old English ge-wis. Here are the listings for the proper name Gewis (itself concocted from the word) and ge-wis from the Bosworth and Toller dictionary:
Gewis, Giwis, es; m. Gewis, the great grandfather of Cerdic :-- Se Cerdic wæs Elesing, Elesa Esling, Esla Gewising, Gewis Wiging Cerdic was the son of Elesa, Elesa the son of Esla, Elsa the son of Gewis, Gewis the son of Wig, Chr. 495; Erl. 2, 5: 597; Erl. 20, 7. Giwis, 552; Erl. 16, 19. According to Asset it was from this name that the term Gevissæ, applied by Bede to the West Saxons, was derived. 'Gewis, a quo Britone totam illam gentem Gegwis nominant,' see Grmm. Gesch. D. S. 458. For the use by Bede, see Bd. 3, 7-'Gens Occidentalium Saxonum qui antiquitus Gevissæ vocabantur ... primum Gevissorum gentem ingrediens,' where the translation has 'West Seaxna þeód ... Ðá com he æ-acute;rest upp on West Seaxum.' See also 4, 15, 16. Smith's note on the word is 'Gevissæ. Saxonicum est pro Occidentalium. Sic Visigothi præposita tantum Saxonica expletiva Ge.' See Thorpe's Lappenberg i. 109, note.
ge-wis, -wiss; adj. Certain, sure, knowing, foreknowing; certus :-- Gewis be heora gerihtnesse certus de illorum correctione, Bd. 5, 22; S. 644, 45. Ðæt is gesægd ðæt he wæ-acute;re gewis his sylfes forþfóre qui præscius sui obitus exstitisse videtur, 4, 24; S. 599, 14. Wite ðæt érest gewiss ðæt ðæt mód byþ ðære sáwle æ-acute;ge know first that as certain, that the mind is the soul's eye, Shrn. 178, 2. Gewis is constat, Hpt. Gl. 419. Ða úþwitan ðe sæ-acute;don ðæt næ-acute;fre nán wiht gewisses næ-acute;re búton twæónunga the philosophers that said that there was no certainty without doubt, Shrn. 174, 25. Swá litel gewis funden found so little certain, Bt. 41, 4; Fox 250, 20. Gewis andgit intelligence, 5; Fox 252, 20, 30. We syndon gewisse ðínes lífes we are acquainted with thy life, Guthl. 5; Gdwin. 30, 18. He hí gewisse gedyde and gelæ-acute;rde be ingonge ðæs écan ríces de ingressu regni æterni certos reddidit, Bd. 4, 16; S. 584, 35. On gewissum tídum at certain times, R. Ben. interl. 48. Of gewissum intingan of certain causes, R. Ben. interl. 63. Myd gewyssum gesceáde with certain reason, wherefore; propter certam rationem, quapropter, Nicod. 3; Thw. 2, 6. [O. H. Ger. giwis: Ger. gewiss certus.]
It has been thought (including by the present author) that the term was meant to be a way of distinguishing "good" Britons from "bad", the bad ones being, of course, the wealas or Welsh. Your enemy is a foreigner and strange to you. Your friend or ally is known and you can be sure and certain of him.
But I've just had cause to wonder whether there might be more behind the Gewissei name - as well as the Cuth- names who are first brought into connection with Ceawlin/Cunedda in the Anglo-Saxon Chronicle. I long ago made a case for the Cuth- names being for the goddess Cuda of the Cotswolds.
Her Ceaulin 7 Cuþa gefuhton wiþ Ęþelbryht. 7 hine in Cent gefliemdon, 7 tuegen aldormen on Wibban dune ofslogon, Oslaf 7 Cnebban.
Her Cuþwulf feaht wiþ Bretwalas æt Bedcan forda. 7 .iiii. tunas genom, Lygeanburg. 7 Ægelesburg. Benningtun. 7 Egonesham. 7 þy ilcan geare he gefor.
Her Cuþwine 7 Ceawlin fuhton wiþ Brettas, 7 hie .iii. kyningas ofslogon, Coinmail, 7 Condidan, 7 Farinmail, in þære stowe þe is gecueden Deorham. 7 genamon .iii. ceastro Gleawanceaster, 7 Cirenceaster, 7 Baþanceaster.
hand8: Her Mauricius feng to Romana rice.
Her Ceawlin 7 Cuþa fuhton wiþ Brettas, in þam stede þe mon nemneþ Feþanleag. 7 Cuþan mon ofslog. 7 Ceaulin monige tunas genom, 7 unarimedlice herereaf, 7 ierre he hwearf þonan to his agnum.
However, this may be wrong. Cunedda/Cunedag /Kynadaf (Cunedaf) of Welsh tradition owes his name to the Irish Chuinnedha, also spelled Cuindedha, Cunnid, Cuinnid. Let us bear this in mind as we look at the various forms of Old English cunnan, ‘know’:
cunnen / (ġe)cūþ
Suppose this happened: the name Cunedda/Chuinnedha, regardless of its meaning in Irish or Welsh, was intrepreted by the early English as being related to their own word cunnan, which has forms such as cunnaþ. And that the Cuth- names themselves should be derived not from the goddess Cuda, but from a spelling like cūþe.
If so - and I realize this is highly speculative, and quite possibly wrong - we might suppose that the term Gewissei came about as a way of identifying the descendants of Cunedda. In other words, the Sure or Certain or Knowing Ones were those belonging to the teulu of a man whose name the English believed meant something akin to the Known One.