This page of the Biomutant guide contains a detailed walkthrough of The Chugyard main story quest.
The Chugyard has always been an important spot for collectors from all tribes, but you can still find plenty of scrap metal here. Gizmo has made his nest underground and so far only the invited ones have managed to get down there.
Head over to The Chugyard - a place marked on the world map. The main gates of the depot will be closed, so you will have to find another way. Walk along the wall to the left and at some point you will notice a passage between the carriages.
Once you enter the Chugyard area, you will have to fight against a large group of enemies, among which there will be one stronger opponent with a battering ram.
After the fight, approach the massive metal door and open it using the crowbar - a button will appear on the screen, which you will have to mash.
There is one opponent stationed at the Sprocket, but it should not cause you any trouble.
After the fight, interact with the rocket and your hero will cling to it and steer it in such a way that the gate leading to Gizmo's hideout will get blown up.