Within a developed worldview, there is often a presentation of cosmological concepts. Not to mention an enumeration of worlds. Though the number varies in various Indo-European cultures, three is a very common number. John Shaw, in ‘On Indo-European Cosmic Structure’ covers this very well. (The link shared is to academia.edu, free to sign up, and many articles are free as well.) Given that many other cultures had a three world cosmological structure at their core, it is not a stretch to assume this to be the case with the Gaulish peoples.
Those three worlds are Albios, Bitus, and Dubnos. A bit of explanation is required.
Dubnos is the Underworld, the Deep, and can relate not only to Mori (Gaulish for “sea”) but that which is under Bitus (the world in which we live). Under lakes, streams, caves, you name it. That which is under the Earth, home to Andernadoi (chthonic) beings. This includes not only Dêuoi of Dubnos, but at least some Rogentiâ (Ancestors), and other spirits. Some benevolent (bestowing riches and fertility of the land), others malevolent. It is here that is the womb of Litauiâ (the Earth), as we come from it, and our bodies, at the very least, return to it.
Dubnos is associated with things Giamos — that is, darkness, chaotic, chthonic, primal, and of winter.
Bitus is our own world, which other than being the home of humanity, is home to a myriad of other beings. This is also the domain of the Litauiatîs (land spirits), which are Dêuoi and Spirits of the Earth. This includes deities tied to locations such as lakes, rivers, mountains, trees, forests, and at this point, if not before, cities. Bitus is acted upon both of the other worlds. Thus we get both order and chaos, to live and die, the turnings of the seasons, and influences from both of the other worlds. We are subject to the full experience of these cycles.
Albios is the upper world. It is home to Ueranadoi (celestial) beings. The Dêuoi and Spirits of Nemos (the Sky), live here. Generally, these are the beings that provide order, protection, and that which is needed for civilization. To simply classify them as benevolent or malevolent does them a bit of disservice, as it is their job to preserve order. However, through reciprocal exchange with Them, as with the Dêuoi of the other two worlds, They too, may return such gifts with Their benevolence.
Albios is associated with things Samos — that is, light, order, celestial, civilized, and of summer.
One can also posit something that links the Three Worlds. That would be Drus (the World Tree). Drus is, in this case, the axis mundi, or pillar of worlds. In this case, comparisons can be made to Yggdrasil in Norse mythology, or Mount Olympus in Greek mythology. The roots of Drus lie within Dubnos, the trunk in Bitus, with a canopy that stretches to Albios.