Composite magnetic structures based on rare-earth/transition-metal (RE/TM) systems are promising candidates for high-density and low-power spintronic devices because their net magnetization, magnetic anisotropy, and spin dynamics can be tailored through composition and interfacial engineering. However, thermal processing may induce elemental interdiffusion and local magnetic inhomogeneity, which can substantially modify both magnetization reversal and high-frequency magnetic response. In this work, Ta (10 nm)/MgO (2 nm)/CoFeB (5 nm)/Gd (5 nm)/CoFeB (5 nm)/Ta (10 nm) multilayers were deposited by magnetron sputtering and subsequently annealed at temperatures ranging from 100 to 500 °C. Their interfacial structures, static magnetic properties, and ferromagnetic-resonance characteristics were systematically investigated by cross-sectional transmission electron microscopy and energy-dispersive X-ray spectroscopy, vibrating-sample and superconducting quantum interference device magnetometry, multi-frequency and temperature-dependent ferromagnetic resonance measurements, and micromagnetic simulations. The low-temperature-annealed samples mainly exhibit CoFeB-dominated ferromagnetic behavior and a single FMR resonance. Cross-sectional EDS analysis shows that annealing at 300 °C broadens the spatial distributions of Gd, Co, and Fe near the Gd/CoFeB interfaces, providing direct evidence of enhanced interfacial interdiffusion. At the same annealing temperature, the initially single FMR resonance evolves into two clearly resolvable branches. Double-peak derivative Lorentzian fitting adequately reproduces the experimental spectra. The two resonance branches can be continuously tracked at different microwave frequencies and shift toward higher magnetic fields with increasing frequency. Separate Kittel fits yield distinct effective magnetic parameters for the two branches, indicating that they originate from two different dynamic response channels rather than from simple linewidth broadening or instrumental phase distortion. Their resonance fields and peak separation vary only weakly with temperature, suggesting that conventional long-range interlayer antiferromagnetic coupling is unlikely to be the dominant origin of the splitting. When the annealing temperature is further increased to 400 and 500 °C, the magnetic hysteresis loops become broadened and more inclined, while the FMR signals weaken and the two branches become less distinguishable. These observations indicate enhanced magnetic inhomogeneity and a more broadly distributed magnetization-reversal process at high annealing temperatures. Micromagnetic simulations based on an effective Gd–CoFeB intermixed interfacial layer reproduce the experimentally observed two-branch dispersion for the 300 °C-annealed sample, whereas a more strongly intermixed model yields broadened and partially merged resonances at higher annealing temperatures. These results establish a direct correlation between annealing-induced Gd–CoFeB interfacial interdiffusion, static magnetic evolution, and the formation of multiple high-frequency dynamic response channels. The central finding is that local magnetic-parameter differentiation induced by interfacial intermixing can generate resolvable FMR mode splitting without requiring dominant long-range interlayer antiferromagnetic coupling. This work provides a physical basis for optimizing the thermal stability and high-frequency dynamic performance of Gd-containing CoFeB composite free layers in magnetic tunnel junction and spin-orbit-torque devices.