A Navi-Gummi is a special Gummi Block, featured in Kingdom Hearts, that allows more worlds to be accessed when installed into a Gummi Ship. Oftentimes, these blocks can be found in halves, which are called Navi-G Pieces. A piece can be joined to its pair to create a fully-functioning Navi-Gummi. Although they are structurally Gummi Blocks, Navi-Gummis and Navi-G Pieces are categorized as key items and therefore, do not count towards the block limit when building ships.
Sora, Donald, and Goofy find two Navi-G Pieces in Wonderland and Deep Jungle. Confused over what these particular Gummi blocks do, the party seek assistance from the Gummi-expert Cid, who explains to them that the pieces are special navigation Gummis that allow Gummi Ships to access new worlds. The mechanic joins the two pieces together to form a complete block, and then installs it into their Gummi Ship, unlocking the orange warp that lets the party travel to the second circle of worlds.
Later, the party return to Cid with two more Navi-G Pieces that they found from Traverse Town and Neverland. As before, Cid combines the two pieces and installed the completed block into their ship, allowing them to reach Hollow Bastion.
When Sora rescues Kairi, he takes her to safety in Traverse Town. However, he had not yet sealed the Keyhole of Hollow Bastion, and could not return there via the normal route due to the high concentration of darkness and Heartless in the area. Cid tells Sora that he had hidden an entire Navi-Gummi behind a mural in the Secret Waterway. Sora fetches it and Cid installs the block on Sora's ship, allowing him to reach Hollow Bastion through a purple warp. Sora and his party return to Hollow Bastion by travelling through this alternative route and seals Hollow Bastion's Keyhole.