For this, In CrossGate, when you level up your Skills, the effectiveness will be increased.
But then you may find out you cant use a certain Level skill.
Your skill is at Lv3 but you cant used it (You are at Lv 15)
You have to be in Lv 20 in order to use a Lv 3 Skill.
(Well, I dunno exactly know if CG2 work like this, but this is the basic concept of CG)
Character Lv 1~9 ==> Skill Lv1
Character Lv10~19 ==> Skill Lv2
Character Lv20~29 ==> Skill Lv3
and so on...
(Same goes as the Equipment, if the concepts are the same)
In Concerto Gate, What happens is when you use the skill. Then you get like exp for using it.
You don't have to be a certain character level to do it.
Thanks for the information though, This'll help :3