f-subshell contains seven orbitals in it. Hence, if seven electrons are present in these orbitals then it said to be half-filled. Electronic configuration of all the elements are:
Sm(Z=62)=4f6s2
Eu(Z=63)=4f76S2
Tb(Z=65)=4f96s2
Gd(Z=64)=4f75d16s2
Pm(Z=61)=4f56s2
So, Eu & Gd have half-filled configuration.