Dear David ~
You appear to have confused two different men named John Gernon, both of whom had a wife, Alice.
The first John Gernon was John Gernon, Knt., of East Thorpe, Essex, Bakewell, Derbyshire, born about 1287/97, died c.1334. He married (1st) Isabel le Bigod. He married (2nd) about 1313 Alice de Coleville. They had one son, John, Knt. He married (3rd) in 1332 Margaret de Wigton. A full account of this family is found in my book, Royal Ancestry (5 volume set), published in 2013.
The second John Gernon, was John Gernon, of Steeple Lavington, Wiltshire. He married by 1302 Alice, daughter of Hugh de Plessis (or Plescy), of Hook Norton and Headington, Oxfordshire, by Isabel, daughter and co-heiress of John Biset. They had one son, John, and one daughter, Katherine (wife of Richard Dauntsey). See VCH Wiltshire 10 (1975): 82-106.
Alice de Plessis (or Plescy) was the sister of Christian de Plessis (or Plescy) (living 1331), wife of John de Segrave, Knt., 2nd Lord Segrave.
Best always, Douglas Richardson, Salt Lake City, Utah