English, from the Welsh Gwenhwyfar, "fair and yielding". Jennifer is a Cornish/Celtic variant of Guinevere, King Arthur's wife.
Gaynor is a medieval variant of Guinevere so we have classed it as a lesser synonym of Jennifer.
Juniper as a given name is possibly associated with Jennifer which ONC suggests was a formative influence on the surname Juniper. We have classed these forenames as "variants" but believe the link to be somewhat tenuous.