To put it simply, it hasn't been explicitly confirmed if Monkey D. Garp, the legendary Marine vice admiral and Luffy's grandfather, possesses the Conqueror’s Haki. While it's known that Garp is incredibly powerful, holding his own against the Pirate King, Gold Roger, and even cornering him multiple times, the exact specifics of his abilities remain somewhat of a mystery. We know he has immense physical strength, and his proficiency with Armament Haki and Observation Haki is highly likely given his rank and combat capability.
Yet, the question about whether he has the Conqueror’s Haki, a form of Haki that only a few individuals in the 'One Piece' world possess, still stands. With Garp's leadership qualities, charisma, and the respect he commands, it wouldn't be surprising if he does have Conqueror’s Haki. It's just a matter of waiting for an official reveal, if there is one awaiting us in the series.
So, for now, fans can only speculate and wait for confirmation from the 'One Piece' creator, Eiichiro Oda. He always has a few tricks up his sleeve, and who knows, a revelation on Garp's abilities might be one of them.