Gallant 5 is a planet somewhere in the LEGO Universe (outside the Nimbus System) that held an ancient structure created by the legendary First Builders. This planet was an important early stop in the Four Explorers' journey, because it gave them the Imagimeter Pieces, which they needed to find the Imagination Nexus. And finding it was the reason that Doctor Overbuild and Baron Typhonus met Hael Storm and Duke Exeter, because only Duke knew where the structure was, and Hael knew how to find Duke. Very little is known about Gallant 5 because the Nexus Force has never explored it, the Faction Leaders being the only ones who know more of it.
It is implied that the clues found by the four explorers on Gallant 5 were the Imagimeter pieces.