Dont worry about the max level right now, the game is still in Open Beta and improving, so the max level will be higher in the future
G-healing and P-healing are two stats that are made mostly for 2 class branches : The Healers and the Tanks. G-healing improves the heal that you give to others (Healers) and P-healing improves the heal that you get (Tanks). These 2 stats work best in combination (when the healer has big G-healing and the tank has big P-healing).
LCK is useful because it contributes to M-crit Rate, M-crit DMG, Crit rate, Crit DMG and ACC. It is really useful with any DPS classes, but you will get LCK on Grimoires also, so dont worry
I am hoping too for the fourth Magic DPS class.. it will probably be the one to use those G-healing Grimoires. Fingers crossed