Paul Bailey (father, deceased); Joey Bailey (twin brother, deceased); Jean Grey (Phoenix) (aunt, deceased); Madelyne Pryor (clone aunt, deceased); Scott Summers (Cyclops) (uncle); Julia Grey (aunt/guardian, deceased); Roger Grey (uncle, deceased); Liam Grey (uncle, deceased); John Grey (grandfather, deceased); Brian Grey (great uncle/guardian, deceased); Elaine Grey (grandmother, deceased); Phyllis Dennefer (great aunt, deceased); Roy Dennefer (great uncle, deceased); Fred Harriman (relative, deceased); Bekka Wallis (cousin, deceased); Julian (cousin, deceased); Mary-Margaret (cousin, deceased); Kindra (cousin, deceased); Derry Campbell (cousin, deceased); Rachel Summers (Marvel Girl) (alternate reality cousin) Nathan Summers (Cable) (cousin, deceased); Nate Grey (X-Man) (alternate reality cousin); Summers and Grey Family Tree (relatives)
Base Of Operations
Place of Death
Gailyn is a daugther of
Paul Bailey and Sara Grey-Bailey, who is the elder sister of Jean Grey. As well as her aunt, Gailyn was a mutant. 
When their house was blown up,
Gailyn and Joey were kidnapped by  Nanny, who brainwash them with her "pixie dust" and force to forget about their parents. Gailyn and Joey were collectively codenamed Shatter-Box and joined the Lost Boys And Girls.
Living with grandparents
They were rescued by Jean Grey and
Cyclops, and then settled with grandparents in Annandale-on-Hudson.  Together with grandparents, Gailyn and Joey were guests at the wedding of Jean and Cyclops. 
Some time later Jean and Scott visited
Grey's House, and Cyclops tried to check children for mutant powers.
It angered Jean, who wanted a happy childhood for the children.
After it became clear that Sara died,
 Julia Grey and Brian Grey become guardians of Gailyn and Joey.
Operation: Zero Tolerance Gailyn and Joey, were hounded by the Prime Sentinels, but were saved thanks to Nate Grey and Roust.
Gailyn was eventually killed, along with most members of the
Greys, by the Shi'ar Death Commandos. 
Powers and Abilities
She was stated to be an Epsilon-Beta Class
mutant by the Prime Sentinels. It is not known what power she had, and whether she retained them after  M-Day.
Nanny's flying spidership.
Their existence was first mentioned when Dark Phoenix read Sara's fears that her children can be mutants as Jean.
 Joey (and therefore Gailyn as his twin sister) was eleven years old, a year after the death of Jean Grey on the Moon.
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