In the Standard Model, some combination of the baryon B and lepton L number symmetry is free of mixed anomalies with strong and electroweak su(3) × su(2) × u(1) Ỹ gauge forces. However, it can still suffer from a mixed gravitational anomaly, hypothetically pertinent for leptogenesis in the very early universe. This happens when the "sterile right-handed" neutrino number nν R is not equal to the family number N f . Thus the invertible B − L symmetry current conservation can be violated quantum mechanically by gravitational backgrounds such as gravitational instantons. In specific, we show that a noninvertible categorical counterpart of the B−L symmetry still survives in gravitational backgrounds. In general, we propose a construction of noninvertible symmetry charge operators as topological defects derived from invertible anomalous symmetries that suffer from mixed gravitational anomalies. Examples include the perturbative local and nonperturbative global anomalies classified by Z and Z16 respectively. For this construction, we utilize the anomaly inflow concept and the 3d Witten-Reshetikhin-Turaev-type topological quantum field theory corresponding to a 2d rational conformal field theory with an appropriate chiral central charge, or the 3d boundary topological order of 4d Z T F 4 -time-reversal symmetric topological superconductor.