Hi, I had a problem with blocky-looking spectra, much like your graphs, doing photoneutron simulations about four years ago Photoneutron process is inaccurate at low gamma ray energy . I was using the ShieldingLEND physics list on Geant4 10.5.1. It was suggested that it might be a bug or peculiarity in LEND. I do not know if it is still a problem under 11.1. I usually use QGSP_BERT_HP or QGSP_BIC_HP for (n,gamma) simulations, so you might want to try one of those.
As for gamma ray line energies and relative intensities, it depends on your geometry (target size and composition, thickness, incident neutron energy, etc.) More information is required to see if it makes sense.