given that a Gecko neck looks like this
and that a 4-string Gecko body utilizes the standard Fender style neck pocket, you're probably looking to use a Wrmoth neck similar to this one
similar headstock to the Gecko ... correct Fender style neck heel ... correct 34" scale. more of these are found here:
http://www.warmoth.com/showcase/sc_guitar_necks.cfm?type=bass&menuItem=5&subMenuItem=2
all the best,
R