Who debuted earlier?
Korea has a strong Confucian culture, and that's why the relationship between seniors and juniors are so tight in Korea, while Kpop idols also follow this kind of rule of society. They bow respectfully to other idol groups, calling them "Sunbaenim"(senior) if the groups debuted earlier than themselves.
By the way, Isn't it strange that SEVENTEEN, who debuted in May 2015 calls iKON, who released their debut album in November 2015 "Sunbaenim"? It is because of iKON's appearance in "WIN" and "Mix & Match." Even though iKON debuted later than SEVENTEEN, iKON began to appear in TV earlier than SEVENTEEN, and that's why SEVENTEEN calls the rookie group of YG "Sunbaenim."
Do trainees have senior-junior relationship too?
Kpop trainees are potential kpop idol stars, and some of them are trained for a very long time. However, all the trainees are just trainees and they do not have senior-junior relationship. No matter how long a trainee was trained, he or she has to call senior idol singers "Sunbaenim."
Why is Seo Inyoung so angry?
In the senior-junior relationship of Korean people, their ages are meaningless.(It's very unique, strange and funny culture of Korea because Koreans are always taught to be very polite to older people)
What Gain said on "Radio Star" was that she felt bad when Seo Inyoung talked roughly to Brown Eyed Girls' Narsha, who is older than Seo Inyoung, just because Seo Inyoung debuted earlier than Brown Eyed Girls. And to this, Seo Inyoung showed her displeasure, commenting, "We have rules in the kpop world." As I've explained above, the rules are that juniors should be very polite to seniors regardless of their ages. Well, I can understand both sides because this kind of thing really happens in Korean daily lives.
Then, who do you think is saying the right thing? Most of old people in Korea might think Seo Inyoung's right because rules are rules. However, times are changing. Young people in Korea might think Gain's right, while many of them are very negative about a strong Confucian culture. That's why Seo Inyoung met severe criticism from Korean netizens.