Hi @ReeceOB.
Yes the mass vector has been extended to match the size of the frequency and atomic coordinates vector. You can simply triplicate the masses for each atom.
Regarding the normal modes, the read_gammes function is an utility function provided to assist the users and it simply reads the output and returns all the normal modes. But 6 of these modes belong to the transnational and rotational degrees of freedom and should be excluded such that you get 3N - 6 vibrational modes. You should selects the modes manually for your molecule but they are typically the last 3N-6 modes. You can visualise the modes with a software such as Jmol that can read your gamess files to be sure. So, please make sure that you properly reshape the normal mode matrices. This post discusses a similar subject and might be useful.
Please let me know if you have any questions.