usually 'er ( 兒 ) after the name is used to indicate that the person has a close relationship , and one generation below right? something like Parents to Children , Teacher/Master to disciple , etc but why Guo Jing called Huang Rong as Rong'er , and Yang Guo changed "Gugu" to Long'er after marriage??
Indeed it is a kind of pet name to express fondness or familiarity. Usually, it's used to call someone younger but doesn't necessarily only for one generation below. Therefore it's appropriate for a husband gave suffix 'er' to call his wife. For Yang Guo called Long-er was for YG to express that he's now 'the leader' in the family instead of he's being younger than XLN
It's more like John is called Johnny when he's a child or a pet name used by his family.