Gul Benil is a Cardassian officer who intercepts the disguised Defiant inside Cardassian space and is talked out of boarding her by Garak, who overawes him with a high-level clearance code.
Benil appears in the third-season episode Second Skin. He commands a Cardassian warship on patrol when the Defiant, cloaked for most of her journey and then masquerading as a Kobheerian freighter, enters Cardassian territory to rescue Major Kira.
Benil hails the supposed freighter and announces his intention to inspect it. Garak, who has been pressed into coming along, takes over the conversation, identifies himself by a security clearance that Benil evidently finds alarming, and warns him that further interference would be unwise for his career.
Benil backs down at once and lets the ship pass. The scene is one of the clearest early indications that Garak's connections to the Obsidian Order were more than those of a plain, simple tailor.