Location: Dressrosa City Area
Petitioner: Blacksmith Garill
Reward: Varies; see below
Objective Reward: Varies; see below

In order to trigger this chain of Side Stories you must first complete the main plot beats of Dressrosa, all the way up to defeating Doflamingo. Once that's done, return to Dressrosa in Chapter 9 and you'll find Side Stories all over the place. The petitioner for this particular Side Story is directly across the street from the Yoisa Sign in Dressrosa City East.

Part 1

Reward: Fairy Pumpkin Gazpacho, 37,000 Berries
Objective Reward: 120,000 Berries

Despite the end of Doflamingo's reign Dressrosa is still plagued by problems, and the petitioner, Garill, wants you to deal with some gladiators who have turned bandit. He asks Usopp in particular to track them down and force them to mend their ways. The gladiators are apparently in Dressrosa City South.

Head to the south-most plaza of Dressrosa City South, down a side street to the east of the Memoria Cube. The gladiators are busy bullying someone by a heap of burning rubble on the east side of the plaza. Usopp will... try... to intimidate them, but ultimately you'll just need to beat them up. All three Warriors are Technique fighters, so Usopp is a decent choice for taking them out. 

After the fight you need to tell the fallen gladiators what to do:
  • Help rebuild the country
  • Trust what a God says
  • Work hard and make them grateful
This will convince the gladiators to turn over a new leaf, and you'll be shot back to Garill to collect your reward. He'll also make Usopp a nice statue, and set it up a few feet away from where he's standing.

Part 2

Reward: Team God Badge, 49,000 Berries
Objective Reward: 120,000 Berries

To continue this Side Story you next need to head to Foot of New Royal Plateau, specifically the side area to the west of N.R.P. Elevator in the southeast of Dressrosa. Out front of the store Apparel Brilliant is a Worried Old Woman, and she has mystery seeds that won't sprout. She asks Usopp to solve her botanical issue.

Jump to the Coliseum Path Yoisa Sign, then check in at Passion, a book store to the northwest of the entrance to the Coliseum. The proprietor, Booksie, will show the group a book with info Usopp needs - specifically, that the seeds require a Bo Bo Bird Egg. There's a better-than-good chance that you already have one in your inventory, but if not, you can use Usopp to shoot down birds' nests around Dressrosa. One will eventually have a Bo Bo Bird Egg in it. (The first one I spotted, attached to the front of the Coliseum, dropped an egg. This search shouldn't take long.)

Take the Bo Bo Bird Egg back to the old woman in Foot of New Royal Plateau. Usopp will explain the process of sprouting the seeds, and the woman will depart for the front of the royal palace, atop the New Royal Plateau. Before you follow her, jump to the camp at the bottom of the New Royal Plateau and rest there until night. Then fast travel up to N.R.P. Sunflower Field and speak to the woman, Liberta, near the front door of the castle.

(Changing the time to night and then fast traveling doesn't always seem to work. Often you'll fast travel and then appear at the Sunflower Field during the day. All you can really do is go back to camp and try again.)

Speak to Liberta and the seeds will sprout, revealing four Ambling Stonecrops. A few bashes from a Power fighter, with a dash of Fire, will do them in. This will end the quest - and Garill will upgrade Usopp's statue.

Part 3

Reward: Team God Crown, three Usopp's Cube Fragments, 74,000 Berries
Objective Reward: Perfected Shocking Ball

To trigger the final step of this Side Story chain you need to head back to Blacksmith Garill. Head north through Dressrosa City East, up the first flight of stairs you see, and look on your left. There's a Well-Dressed Man standing outside a house with some guards, and he wants to speak to Usopp. 

Agree and he'll identify himself as Gramoo, a butler for a distinguised family in Dressrosa. The butler's employers are being persecuted for following Doflamingo in the past, and they want Usopp to convince the, uh, 'poorer' folk of Dressrosa to stop being so mean.

Head down to Dressrosa Main Street, the eastbound road that stretches away from Square, At Fountain. A ways down the street you'll find a group of Ambush-Planning Men who are... well, you know. Planning. You'll need to take them out. The majority are weak to either Speed or Power, and despite their numbers they shouldn't give you a ton of trouble - though given that they call in reinforcements, the fight might take a little while.

And then...

Bodyguard Giant Clan

Weakness: Technique

He's big, but... don't worry about it. Your Technique fighters can take this giant down pretty easily.

Once the fight's over Usopp will need to perform one more round of charismatic conversation to end the quest:
  • You can't hate those who suffered like you did
  • You should be working together to rebuild

That'll about do it. Report back to Gramoo in Dressrosa City East to earn Usopp his reward for a job well done, and to end the Side Story. Go enjoy Usopp's terrible gold statue.