Gewisse

The Ġewisse (; ) was a tribe or clan of Anglo-Saxon England, based in the upper Thames region around Dorchester on Thames.
Etymology.
The name of the tribe may be derived from an Old English word for "reliable" or "sure", (cf. German "gewiss", "certain, sure").
The "Anglo-Saxon Chronicle" presents an eponymous ancestor figure, named "Giwis".
Eilert Ekwall proposed that the similarity in toponymy between the kingdoms of the Gewisse and Hwicce suggests a common origin.
History.
Evidence of Germanic settlements appears around Abingdon and Dorchester on Thames in the 6th and 7th centuries. It suggests the Gewisse may have been mercenaries settled in the region after the end of the Roman occupation to protect a border region between Britons.
The Gewisse captured "Searobyrig" (Old Sarum) in 552 and "Beranbyrig" (Barbury Castle) from the Britons in 556. Birinus converted the Gewisse to Christianity in 636 by baptising their king Cynegils and establishing the Diocese of Dorchester. The Gewisse killed the three sons of Sæbert of Essex in about 620, defeated the Britons at the Battle of Peonnum in 660 and by 676 had sufficient control over what is now Hampshire to establish a see at Winchester.
The conquests by the royal house of Gewisse in the 7th and 8th centuries led to the establishment of the Kingdom of Wessex, and Bede treated the two names as interchangeable. It was only during the reign of Cædwalla (685/6 – 688) that the title "king of the Saxons" began to replace "king of the Gewisse". Barbara Yorke has suggested that it was Cædwalla's conquest of the Jutish province and the South Saxons that led to the need for a new title to distinguish the expanded realm from its predecessor. However, as there are no surviving documents to indicate how these people described themselves, the most that can be said is that by the time Bede was writing (early 8th century), the phrase "West Saxons" had come into use by scholars.