Just to add to the excellent posts.... Sometimes, some geldings are just more "boy like" in their head than others. All geldings are hardwired as stallions, they just no longer have the drive to breed and behave like stallions all the time. This means some of them can be pretty pushy - even if they are subtle about it.
One trainer I know puts it this way "A gelding is a stallion with the volume turned down"
With this in mind, it might explain WHY your gelding is testing testing testing... What he is really asking for is for you to be a stronger leader. No safer place for a horse, any horse, than with a strong leader... The boys seem to feel a need to question leadership more often than the girls.
Beyond that, I see some great suggestions here about how to go about becoming that stronger leader!