A good survey of the Gelert and related legends is to be found in "The Holy
Greyhound; Guinefore healer of children since the thirteenth century", Jean
Claude Smit, Cambridge University Press, 1983 (Translation).  Well worth
reading if you want to know all the versions of the story even the saintly
details...

Dr H. La Rue,
University Lecturer/Curator of the Bate collection
and the Musical Collections of the
Pitt Rivers Museum,
Oxford