  We show that a Gibbs characterization of normalized generalized Gamma
processes, recently obtained in Lijoi, Pr\"unster and Walker (2007), can
alternatively be derived by exploiting a characterization of exponentially
tilted Poisson-Kingman models stated in Pitman (2003). We also provide a
completion of this result investigating the existence of normalized random
measures inducing exchangeable Gibbs partitions of type $\alpha \in (-\infty,
0]$.
