  In this work, we consider a family of Gelfand pairs $(K \ltimes N, N)$ (in
short $(K,N)$) where $N$ is a two step nilpotent Lie group, and $K$ is the
group of orthogonal automorphisms of $N$. This family has a nice analytic
property: almost all these 2-step nilpotent Lie group have square integrable
representations. In this cases, following Moore-Wolf's theory, we find an
explicit expression for the inversion formula of $N$, and as a consequence, we
decompose the regular action of $K \ltimes N$ on $L^{2}(N)$. This result
completes the analysis carried out by Wolf, where the inversion formula is
obtained in the case that $N$ has not square integrable representation. When
$N$ is the Heisenberg group, we obtain the decomposition of $L^{2}(N)$ under
the action of $K \ltimes N$ for all $K$ such that $(K,N)$ is a Gelfand pair.
Finally, we also give a parametrization for the generic spherical functions
associated to the pair $(K,N)$, and we give an explicit expression for these
functions in some cases.
