It was a hypothetical comment. I don't see it as a realistic outcome.
In any case, just for the sake of argument, Felipe VI was the first king of Spain who did not marry another royal. It is unclear whether his marriage to Letizia sets a precedent for the future or not, but a dynastic marriage for Leonor is still a possibility and, among the potential candidates, Gabriel of Belgium would be one of the best matches.
Besides, Gabriel will be the most senior agnate in the Belgian Coburg dynasty when his father passes away, but nonetheless will not inherit the throne of the Belgians due to the introduction of equal primogeniture. Becoming the consort of the future Queen of Spain and having his descendants sit on the Spanish throne would be fair for him as compensation and would also provide Spain with a proper new Bourbon-Saxe-Coburg dynasty to replace the current Bourbon-Anjou house after Leonor. It is really a win-win situation for the two royal houses.
I see. We'll see what the future brings. I think it a likely scenario that he will end up with a girl from Belgium. Somehow.