The root of the word can be divided in two:The first root, Gil, can be either from the old norse "geisli" that means "pole" (part of a weapon), from the langobardic "gisil" that means "shaft of an arrow", or from the ancient germanic "gisal(a)" that means "pledge", "hostage" or "descendant".The second root, Bert, can be either from the old norse "bjartr", from the old german/old saxon "beraht", or from the gothic "bairht", all of them means "light", "bright" or "shining"Sources:https://www.nordicnames.de/wiki/Gilbert https://www.nordicnames.de/wiki/Gisilberht https://www.nordicnames.de/wiki/GISL https://www.nordicnames.de/wiki/BJ%C3%96RT.
The first root, Gil, can be either from the old norse "geisli" that means "pole" (part of a weapon), from the langobardic "gisil" that means "shaft of an arrow", or from the ancient germanic "gisal(a)" that means "pledge", "hostage" or "descendant".
The second root, Bert, can be either from the old norse "bjartr", from the old german/old saxon "beraht", or from the gothic "bairht", all of them means "light", "bright" or "shining"
Sources:
https://www.nordicnames.de/wiki/Gilbert
https://www.nordicnames.de/wiki/Gisilberht
https://www.nordicnames.de/wiki/GISL
https://www.nordicnames.de/wiki/BJ%C3%96RT.