  We probe the spin Seebeck effect in Gd$_{3}$Fe$_{5}$O$_{12}$/Pt hybrid
structures as a function of temperature and observe two sign changes of the
spin Seebeck signal with decreasing temperature. A first sign change occurs at
a temperature close to the Gd$_{3}$Fe$_{5}$O$_{12}$ magnetic compensation point
at around 280 K. There the spin Seebeck signal changes sign abruptly with
unaltered amplitude, indicating that the spin current is mainly caused by the
magnetic Fe sub-lattices, which reorient their directions at this temperature.
A second, more gradual sign change takes place around the ordering temperature
of the Gd sub-lattice in the range of 65-85 K, showing that the Gd magnetic
sub-lattice dominates the thermally driven spin current at lower temperatures.
These sign changes together with the non-monotonous dependence of the spin
Seebeck signal on the temperature demonstrate that the magnonic spin current is
not simply replicating the effective magnetization of Gd$_{3}$Fe$_{5}$O$_{12}$.
Rather, the thermally generated net spin current results from a complex
interplay of the three magnetic sub-lattices involved.
