How to calculate molecular emission spectra?
As you mentioned, Gaussian can do it but is proprietary.

I believe ezSpectrum (https://github.com/iopenshell/ezSpectrum) might be able to do what you are looking at but I have not used it myself so far.

There used to be a program available called FCClasses which would run the Franck-Condon computations for you based on (I think) Gaussian output, but it seems like the author took it down, I can only find a front-end to it right now: https://github.com/jcerezochem/fcc_tools The FCClasses homepage is mentioned as http://www.pi.iccom.cnr.it/fcclasses on that page but, as mentioned above, no longer works.