Garp is one of the major figures, along with Rocks, Shiki, Sengoku and Whitebeard, from the times when Gol D. Roger, the “Pirate King”, was still alive.
Even at the ripe age of 78, Garp is still on of the strongest characters in the series.
In Chapter 1080 he infused his fist in Conqueror’s Haki and attacked Hachinosu with Galaxy Impact and decimated the surrounding area. That was one of the most devastating attacks ever seen in the series.
What’s even more impressive is that Garp’s punch didn’t even physically touch the terrain. He was indeed a hundred meters above the island, if not even more. Still, he generated such destruction with a shockwave created by his immense Haki.
Now it all makes sense. At the end of the fight between Garp and Sengoku against Shiki, some Marines said “get the vice admiral out of here”. It was not because Garp was injured, but because he already destroyed half of the island (even the narrator said that by the end of the fight, half if the island was destroyed).
Back in the Marineford war, when Garp attacked Marco out of nowhere, Sengoku was angry at Garp for attacking. That was because he was afraid for the safety of Marineford.
And that also explains how him and Roger beat Rocks D. Xebec, Big Mom, Kaido, Shiki and Whitebeard.