Gnon approves of colonialism and condemns immigration. Slavery is a kind of immigration.
In immigration, an inferior society exports its components to a superior society. This will dilute the superior qualities of the superior society, and often decrease Malthusian pressure on the inferior society, delaying its eventual failure.
In colonialism, a superior society replaces an inferior society, hastening its failure and spreading the successful institutions of the superior society. A caveat: one should be careful to colonize in a way consonant with property rights.
Slaves are taken from an inferior society to a superior one. At best, components of the inferior society are preserved contingent on the superior society's continued existence. At worst, the slaves eventually join the gene pool. Athens, thy fool.
Taking sterilized slaves is therefore fine. There's a natural expiry.
Similarly, while there's nothing contrary to property rights in selling one's self into slavery, a wise society may none the less fail to insure such a trade, as it prolongs the inevitable failure. Better the would-be slave starves or kills themselves.