The Jambon-Gruyère is the ham baguette built specifically around Gruyère, and the choice of that cheese rather than the milder default is the point. Gruyère is a pressed cooked-curd cow's-milk cheese, aged until it turns firm and dense with a deep, faintly grainy savor and very few or no holes. Set against the more common Emmental, it is saltier, drier, and more assertive, with a longer finish. A counter that specifies Gruyère is telling you the sandwich is pitched toward depth rather than neutrality.
The build is a baguette, slices of jambon de Paris, slices of Gruyère, and often a thin spread of butter. The logic is contrast held just short of clashing. The boiled ham is pale, soft, and gently sweet; the Gruyère is firm and savory and lingers after the ham has faded. That gap is what gives the sandwich its character. The butter, when present, is kept thin because the Gruyère already supplies salt and richness, and a heavy layer would blunt the edge the cheese is there to provide.
The craft is in the slicing and the balance. Gruyère cut into slabs turns the sandwich dry and overwhelming, since an aged cheese with little moisture does not yield the way a young one does; sliced thin it stays supple and reads as part of the filling rather than a wall of cheese. The ham and Gruyère want to be in rough proportion so the sandwich is one thing rather than alternating strong and mild bites. The baguette still has to be fresh, the crust still has to crack, and the sandwich is still best eaten soon after it is made.
Variations move along the firm-cheese rack: a young Comté for more complexity and a softer edge, an aged Comté for even more grip, an Emmental for the milder everyday reading. Each trades some of the Gruyère's salt-driven assertiveness for a different register. All of them are the same sandwich at the base, the Jambon-Beurre carrying a firm aged cheese alongside the ham.