Spin dynamics in Al-substituted yttrium iron garnets is investigated using broadband ferromagnetic resonance measurements in the temperature range T = 200-360 K. Using the measured data, the resonance field and linewidth, as well as their temperature dependence, are determined, with implications for the uniformity and overall quality of the samples prepared via different chemical fabrication routes. These key parameters governing the spin dynamics in the material are important for its applications in high-speed spintronic and magnonic devices.