Monkey D. Garp is a “Hero” of the Marines. He got the distinction of “Hero of the Marines” from dealing with Rocks, a group of people who were prominent 40 years ago before the Pirate King Gol D. Roger came into power.
Garp is known for cornering and fighting Gol D. Roger many times. Roger also stated that he and Garp had almost killed each other on numerous occasions.
As a vice admiral, Garp is fully capable of using Haki. He demonstrates proficient usage of Busoshoku Haki, being able to injure Luffy several times, bypassing his rubber body. He was also seen crushing Chinjao’s uniquely shaped head with a Haki imbued punch 30 years ago, by hardening his fist and turning it obsidian black to increase the damage inflicted.
During the Battle of Marineford, he managed to punch and hurt Marco despite the latter being in his regenerative phoenix form.
Aside from his incredible mastery over Busoshoku Haki, Garp is also very knowledgeable of the other two types. It was recently revealed that Garp possesses the ability to use Kenbunshoku Haki. When Koby awakens Kenbunshoku Haki, Dr. Fishbonen recommended that Koby ask Garp how to master it, suggesting he has enough expertise in it to mentor others.
Although not demonstrated to be a user of Haoshoku Haki, Garp commented that Luffy has “inherited” Haoshoku Haki when it was subconsciously released.