  The magnetic, thermodynamic and electrical/thermal transport properties of
the caged-structure quasi-skutterudite Gd$_3$Ir$_4$Sn$_{13}$ are
re-investigated. The magnetization $M(T)$, specific heat $C_p(T)$ and the
resistivity $\rho(T)$ reveal a double-phase transition -- at $T_{N1}\sim$ 10~K
and at $T_{N2}\sim$ 8.8~K -- which was not observed in the previous report on
this compound. The antiferromagnetic transition is also visible in the thermal
transport data, thereby suggesting a close connection between the electronic
and lattice degrees of freedom in this Sn-based quasi-skutterudite. The
temperature dependence of $\rho(T)$ is analyzed in terms of a power-law for
resistivity pertinent to Fermi liquid picture. Giant, positive
magnetoresistance (MR) $\approx$ 80$\%$ is observed in Gd$_3$Ir$_4$Sn$_{13}$ at
2~K with the application of 9~T. The giant MR and the double magnetic
transition can be attributed to the quasi-cages and layered antiferromagnetic
structure of Gd$_3$Ir$_4$Sn$_{13}$ vulnerable to structural distortions and/or
dipolar or spin-reorientation effects. The giant value of MR observed in this
class of 3:4:13 type alloys, especially in a Gd-compound, is the highlight of
this work.
