I would like to know whether a baryonic mass defined as "mass_gas" + "mass_stars" using the parameters in the API version (for example in https://www.tng-project.org/api/TNG100-1/snapshots/99/subhalos/461864/) includes ionized gas mass.
Yes it does: this would be all in "mass_gas". You could obtain the ionized or neutral hydrogen mass from the gas using the particle data in the snapshot and the NeutralHydrogenAbundance field.
Are you implying that the ionized gas mass and the neutral hydrogen gas mass are the same?
Rather, that ionized_H_mass + neutral_H_mass = total_H_mass =~ 0.75 * total_gas_mass.
ionized_H_mass + neutral_H_mass = total_H_mass =~ 0.75 * total_gas_mass
Thanks for the clarification; that makes sense.