The equivalent to the Ham City antenna that would work fine for a GMRS repeater is the Comet GP-9N "commercial" version. It covers 460-470MHz and 155MHz. You want a fairly high performance antenna on a repeater and a J-pole doesn't fit that category.
prcguy
SkipSanders said:
Or the massively cheaper GMRS J-Pole made by KB9VBR: