Giselbert would be an ancient form of
Gilbert.
Florent is a form of
Florence or Florenz from the southern Netherlands.
Gosewin is a form of Goswijn, a compound name which goes back to Germanic roots meaning "Goth" (the ancient Germanic tribe) and "friend".
Gerlof goes back to Germanic roots meaning "spear" and "wolf".
Claes would be an alternative spelling of
Klaes, a Frisian short form of
Nicholas.
Renger goes back to a compound Germanic name from
regin, "advice, counsel" and
ger, "spear".
The above information all comes from
Woordenboek van Voornamen by J.
Van der Schaar.