The lowest value of n will be 5 for which g orbital can exist
See in azimuthal quantum number if n = some integer say 1 then L= 0 to n-1
N = 1 -----' L = 0 means only 1 subshell
N = 2 ------- L= 0,1 means only 2 subshell
likewise,
N = 5 ----'' L= 0,1,2,3,4 so here the maximum value of L is 4 means g orbital can exist means g orbital can only exist when L is having a value of 4.