I haven't tried it so I'm not sure, but I think I heard it gives your pet a random gender (so your genderless pet could end up either male or female, you don't pick).

I tried to re-find a thread that was similar to this I seen a while back but the closest I could get was this one here hope it might help?

I read somewhere on another thread that it changes to male first. I haven't tried it myself, though, so I'm not completely sure.

Edit: Pretty sure it does always change to male. I just used one on Seltia, who was genderless, and now he's a male. Also, sorry for double-pinging you.