We combine ferromagnetic resonance absorption measurements (FMR) with spin pumping measurements

to ascertain the full magnetization dynamics of Y3Fe5O12 (YIG)/Gd3Fe5O12 (GdIG)/Pt heterostructures. This

trilayer system offers the unique possibility to individually investigate the spin dynamics of the ferrimagnetic

GdIG close to its compensation temperature. We show that this trilayer acts as a highly tunable spin current

source and our experimental results are corroborated by micromagnetic simulations. The detected spin current in

the top Pt layer is governed by spin dynamics in the GdIG layer, while the broadband FMR absorption spectrum

of the full heterostructure comprises contributions from spin dynamics in both layers. Thus, combining the

measurements of FMR absorption and spin current generation from the spin pumping and spin Seebeck effect

allows us to understand the spin dynamics contributions of both constituents.