The Gummi Ship is really needed in Kingdom Hearts 3 since it allows you to travel between worlds, but in order to be able to reach your destinations, you will need to level it up along the way. The game itself does not really do a very good job of explaining how to level up your Gummi Ship, but don't worry, we got you covered!
When you level up your Gummi Ship, it gains additional benefits, including things like stronger attacks, or mini ships that will also attack enemies. It certainly makes it a lot easier to travel in the Space in Between with some extra levels on your Gummi Ship.
There are four different ways to get Experience Points for the Gummi Ship, which will eventually lead it to level up. These are as follows;
1) Collect the floating orange orbs.
You will see a lot of floating orange orbs while you travel in space, and these are in fact experience orbs. So you will want to pick up as many of these as you can, and your ship will gradually become more powerful.
2) Defeat enemies with the Gummi Ship.
All defeated enemies also give you a certain amount of bonus EXP for your Gummi Ship, so you can essentially grind levels by flying around and killing enemies from your ship. So make sure to kill all enemy spacecrafts if you want to make your Gummi Ship more powerful!
3) Opening up Treasure Spheres.
Treasure Spheres are located all around the space, and you should shoot these to begin a mini game that eventually unlocks them. These spheres will contain a huge chunk of EXP for your Gummi Ship, and in additional they also give you nice loot that you definitely want to pick up. So open these whenever you come across them, or go to your map to see where you can find more Treasure Spheres.
4) Win boss battles with the Gummi Ship.
There are a few boss battles fought with the Gummi Ship, and these also give a good amount of Experience Points upon defeat. So don't be afraid to tackle some bosses to get some extra EXP for your Gummi Ship.