The saint you must be referring to is St. Gerard Majella. Majella has been used as a feminine name in his honor, especially in Ireland. But I'm not sure that would be the origin of Magel or Majel.
From Googling, Magel seems to be a German surname. However, it's not listed in my available reference books, the closest being Mägerl, which is a German name going back to Latin macer, "thin, delicate," according to Hanks & Hodges's A Dictionary of Surnames.