Remember, too, that Gand don't take names until they do something worth gaining a name. Otherwise they just go by "Gand."
I think they have them even if they don't generally use them. I'm sure the bounty hunter license of a Gand hunter isn't just made out to "Gand" nor would the BoSS papers for a starship just list "Gand" on them. The Gand may just refer to himself simply as "Gand", but that's not the same as not having a name.
Any Gand who has made it off planet has completed an achievement worthy of at least one name.
Does that include those that were born off planet? Is surviving birth a "worthy achievement" (some kids today seem to think so)? If not, I'd be wary of such generalizations.
Considering the environmental requirements for Gand, I don't really imagine a lot of them leaving planet unless they've proven themselves capable. These environmental requirements would also severely limit the number of Gand born away from the planet. So while there may be some Gand somewhere who was born off planet, that's a rare enough occurrence to me that some unique situational stuff is being applied.