many people when they put in the name of the lion or its description always put "G1" or "G5" and variants of that type.

For example, I have seen cases of: "Primal G7|Albino|Stats 650"

I see that word a lot when I'm looking at businesses in the Trading Center, especially when they are mutants or hybrids.
What does it mean? Is it something important at the time of breeding or something like that?