Hello everyone,
Gale will appear in Defiance from episode 6 "Brothers in Arms", it was confirmed on Twitter by the executive producer of the show Kevin Murphy!
According to tv.com the episode is scheduled for May 20th.

@chadmom63 gale Harold begins his #defiance arc in episode "Brothers in Arms" ep 6 or 7, depending if you count pilot as 1 or 2 eps.

— Kevin Murphy (@kevinmurphyhc) 30 aprile 2013

Thanks to Klaudia62!