Monkey D. Garp is a lifetime colleague of Sengoku’s, having occasionally accompanied on his assignments when he was still an admiral, and even then Sengoku found him irritating.
As Garp is considered a “Hero of the Marines”, Sengoku, along with the rest of the Marines, respects him greatly. However, their relationship was tested after discovering that Garp withheld the information that the infamous Monkey D. Luffy is his grandson, which left the Fleet Admiral furious at Garp. This added to the frustration caused by Luffy’s repeated actions against the World Government, Sengoku started losing patience with Garp. Garp, on the other hand, took the development happily much to Sengoku’s nerves.
Sengoku had somehow found out that Garp had adopted Portgas D. Ace, the son of Pirate King Gol D. Roger, which was meant to be kept secret from the World Government. He warned Garp not to do anything foolish regarding Ace’s execution, or there would be no mercy.
Sengoku also knows that the Revolutionary known as Dragon is Luffy’s father, and by extension Garp’s son. It is unknown as to how long Sengoku knew about all this, or when he first learned about this, or how it would affect Garp’s standing in the Marines. However, Sengoku had no problem in revealing everything to the entire world.
When Garp allows Luffy to pass him, Sengoku was displeased, saying that Garp was in the end “still a grandfather” and when Garp stood up after being hit by Luffy, he asked him whether he was alright, either showing concern or resentment but it is unknown how this will affect their relationship; while other Marines were shocked when Luffy defeated Garp, Sengoku was one to see that Garp intentionally allowed Luffy to win. Sengoku is also quick to restrain Garp if he tries to do anything foolish as shown when Sengoku pinned Garp to the ground when he later tried to go after Akainu for killing Ace.