This word has given rise to a host of guesses:
Railey suggests garrula, a chatterbox.
Minshew ventures the Italian girella, a weather-cock.
Skinner goes in for the Anglo-Saxon ceorl, a churl.
Why not girdle, as young women before marriage wore a girdle [girle]; and part of a Roman marriage ceremony was for the bridegroom to loose the zone.
As for guessing, the word gull may put in a claim (1 Henry iv. 1); so may the Greek koure, a girl, with a diminutive suffix koure-la, whence gourla, gourl, gurl, girl.
(The Latin gerula means a maid that attends on a child. Chaucer spells the word gurl.)
Probably the word is a variation of darling, Anglo-Saxon, deorling.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894