The short answer is seaweed and water, in that order of importance. The longer answer is why a material drawn from nori behaves on skin the way nothing else does, and why no oil, lotion or soap substitutes for it.
Ask what the gel is and the honest answer is short: seaweed and water, in roughly that order of importance. The material is drawn from nori, the red algae that is also dried into sheets for the kitchen, and what is taken out of it is a long-chain sugar, a polysaccharide, that behaves in water in a way very few substances do. A small quantity of the powder will thicken a very large quantity of water. What comes out of that is not a syrup and not a cream. It is a clear, heavy, slow liquid that holds its own shape for a moment before it decides to move.
That is the whole of it. A gel worth using has no perfume, no colour, no flavour and none of the actives a cosmetic would carry, because every one of those would give the material a second job to do. Strip all of it out and what is left is defined almost entirely by its physics, which is the only part that matters once it is on skin.
An oil is a lipid, and part of the point of an oil is that it is absorbed. It sinks, it thins as it warms, and after twenty minutes a therapist working with it is reaching for more. The gel does the opposite. It is water-based and it does not go into the skin at all; it sits on the surface and stays there, and because it is mostly water it carries heat the way water carries heat, which is to say generously and for a long time. Poured warm, it stays warm long past the point at which an oil would have gone cold and tacky.
The other difference is thickness. Oil is thin enough that two bodies pressed together still catch on each other in small ways, and those small catches are what a conventional stroke is built out of. The gel is thick enough to hold a genuine film between two surfaces, and a film that will not break is a film with almost no friction across it. That is the entire technique in one sentence: the material abolishes friction, and the work is what you can do once friction is gone.
People assume the gel is slippery in the way soap is slippery. It is not, and the distinction is worth having. Soap is slippery because it is a surfactant and it is constantly trying to leave, taking the water with it. A soapy massage is a technique of lather and rinse and lather again, and it belongs in or beside water. The gel belongs on a warm dry surface and it stays where it is put. It will still be doing its work an hour after it was poured, at the same weight and the same temperature as the room, and it is that persistence rather than any dramatic slickness that makes the long stroke possible.
It is also heavier than people expect. A litre of it has real mass, and when it is spread across a back it can be felt as weight before it is felt as anything else. Therapists trained on it work with that: the pressure in a nuru massage comes from leaning rather than pushing, and the gel is part of what transmits it.
It is not a cosmetic and it does not claim to do anything to skin. It is not flavoured or scented, and a gel that arrives smelling of anything has been adulterated for the wrong reasons. It is not a lubricant sold by the sachet, which is a different material at a different viscosity doing a different job. And it is not permanent: it is water-soluble, so a shower takes it off completely, which is why the session ends where it does.
None of this is exotic once it is said plainly. The technique has a reputation for mystique that the material does not really support. It is seaweed, water and heat, handled by someone who understands what those three things do together, and the rest of the hour follows from that. If you want the practical version rather than the chemical one, the what-to-expect guide covers the room, the sheet and the order in which things happen.