cos1776 Note of Caution: The first name seems to be in question.
1861: Archaeologia Cambrensis1 calls her "Margaret".
2012: The Peerage2 calls her "Sybil (?)" with a question mark.
References
Margaret, in Archaeologia Cambrensis - Third Series No. XXV January 1861 p 16.
... Sir John left Sir John [St. John], who married Margaret daughter of Morgan ap Jenkyn ap Philip, and had : 1, Sir John : 2, Sir Oliver, of Shernbrook, Beds : 3, Alexander : and five daughters, of whom the fourth, Margaret, married Sir Rowland Gamage of Coyty. ...
[Last accessed, 26 May 2014. Last Edited, 30 Oct 2012]
Sibyl (?) is the daughter of Morgan ap Jenkyns ap Philip.2 She married Sir John St. John, son of Sir John St. John and Alice Bradshaigh.1 Her married name became St. John ----- [citing Mosley, Charles, editor. Burke's Peerage, Baronetage & Knightage, 107th edition, 3 volumes. Wilmington, Delaware, U.S.A.: Burke's Peerage (Genealogical Books) Ltd, 2003. volume 3, page 3484]