  We apply the generator coordinate method (GCM) to single-$\Lambda$
hypernuclei in order to discuss the spectra of hypernuclear low-lying states.
To this end, we use the same relativistic point-coupling energy functional both
for the mean-field and the beyond-mean-field calculations. This relativistic
GCM approach provides a unified description of low-lying states in ordinary
nuclei and in hypernuclei, and is thus suitable for studying the $\Lambda$
impurity effect. We carry out an illustrative calculation for the low-lying
spectrum of $^{21}_\Lambda$Ne, in which the interplay between the hypernuclear
collective excitations and the single-particle excitations of the unpaired
$\Lambda$ hyperon is taken into account in a full microscopic manner.
