As I just replied in another thread, G_mmpbsa analysis - #2 by MagnusL, there are not many gmx_mmpbsa experts here.
I haven’t used the gmx_mmpbsa tool at all myself, but in general, when you analyse delta G you always need to know what you are comparing to. If not having a specific reference state, it is common to set the lowest delta G to 0. You could then consider delta G to be delta delta G relative to the most favourable configuration. However, if many frames have exactly delta G 0, that is suspicious.