So the system is effectively 2D, not 1D, hence the band structure also has a non-trivial dispersion in the Y direction. You should re-compute the band structure to also include this direction, say by doing G,Y,Z,G, although more properly one should try to use symmetry points of the underlying hexagonal lattice (G,K,M, in 2D), which can be a bit tricky because they are not defined for the square supercell you have created.
If you're only looking for a confirmation that this is a metallic system you can do that, there should be bands on G-Y that cross the Fermi level. On the other hand this fact almost doesn't need proving, since you basically just have pure graphene, which we know has a zero band gap at the K point, with a point defect, which is not enough by itself to open any real band gap.
If you need an accurate band structure, then I suggest creating the system a bit differently, namely by making a supercell by repeating the fundamental graphene hexagonal cell a few times, before introducing the defect. The conversion to supercell means the cell goes to UnitCell, and you lose the symmetry points, but in the Builder you can convert it back to Hexagonal, and then you can easily set up the band structure along the usual routes, like G,K,M,G.
