c++ – Which GOP’s (M and N) values can be used in the video encoded with any MPEG?

I faced the problem with defining either GOP is right or not after parsing the MXF container. I checked the actual GOP value using FFProbe and MediaInfo and these values differ (E.g. M3N15) from which i get with help of MXF 5.2.2 3rd-party library in the code (M4N15). Actually, i created so-called mpeg-calculator just in case some values are not extracted with MXF library. However, i’d like to know all possible gop’s cases that are valid and can be existed in terms of mpeg encoded video.