Each character will have a set of side quests that you are going to complete for them if you want 100% completion of Code Vein.
Gustav Side Quest Guide
Request 1: Dried-Up Trenches – Dried-Up Trenches Entrance
After you have killed the boss, enter the Dried-Up Trenches to interact with Gustav at the Entrance. He is going to ask you for a document that is a piece of evidence to prove that Silva’s policies are a mistake. You have to head to the Ruined City Center – Parking Garage mistle and go over to the third floor in order to retrieve the evidence, Citizens’ Opinions. Go and give it to Gustav in order to complete the side quest.
Request 2: Cathedral of the Sacred Blood – Outlook Tower
Go to the Outlook Tower in order to interact with Gustav and take the stairs. Take left and you are going to find the Damage Report here. Go back and give it to Gustav in order to complete the side quest.
Request 3: Ridge of Frozen Souls – Observatory Site
Gustav will ask us to retrieve another item for him from the Observatory Site. Head straight and at your right, you are going to face an enemy. Defeat him and get the item, Project Evaluation. Go and give it to Gustav in order to complete the side quest.
Request 4: Ashen Cavern – Accumulator Ruins
You are going to find Gustav near the Accumulator Ruins mistle. After your interaction with him, you are going to get the Depth map to Arachnid Grotto. Go to Davis at the Home Base in order to access the location and go defeat the monster at the red cross. Head back to Gustav to inform him about this and complete the side quest.