First off let me say I am very against probing in lizards-if done improperly or done at all to certain species you can really injure the animal, and in lots of lizards it is not an accurate way to determine sex.
With G. validus, the only visible cues are a slightly smaller size for adult females, and males during breeding can get a bit of slight pink on their chin and stomache. Other than that the sexes appear identical.
They arent very antagonistic towards individuals of the same sex in an appropriately sized enclosure though, so sex determination isnt really that important. If you want to breed them get a group of them, its going to be the only reliable way to get both sexes. But for God's sake stop probing them.
Paul