I can't see him as a dark knight, especially considering that he never phisically attacks. So it would be easy to classify him as a black mage, because he does fit that description, and summoner because he does that once, but I like to look back at the original FF sometimes and what do black mages evolve into? Black wizards. Besides, sorcerers can use both white and black magic, but Golbez is never shown using any type of white magic. Rydia is considered to be a summoner with black wizard tendencies, but I would consider Golbez (who gets beaten by the appearance of Rydia) to be the yang to her yin. So a Black Wizard with summoning abilities.