Gilbert, as in
Gilbert and
Sullivan, appeals to me greatly: it's a surname of course, in his case. Otherwise, it's very
Anne of Green Gables.
Giltbert would be OK in a (scholarly) historical novel.
Jeltje is fine in the Netherlands; female, of course.
I like
Gareth,
Gavin and
Gwilym; for a girl, only
Georgina really. And then I'd shorten it to sound like
Jean, so there doesn't seem to be much point.