Gd and its alloys are good magnetic materials, and the addition of the transition metal Co to Gd-Ti alloy can facilitate the formation of thermally stable magnetic compounds. The Gd-Co-Ti alloy exhibits significant potential for the development of magnetic in situ composite materials. However, because of the large positive mixing enthalpy between Gd and Ti, the Gd-Co-Ti alloy is a typical monotectic alloy, exhibiting a miscibility gap in the liquid state. Under the ground gravity conditions, the alloy tends to form a phase-segregated solidification microstructure resulting from liquid-liquid phase transformation. Strong convection in the melt during solidification aggravates this process, rendering it difficult for various influencing factors to interact. However, research on solidification theory for these alloys is limited. Microgravity environments can effectively weaken or even eliminate natural convection in alloy melts, which is beneficial for studying the solidification process and microstructure formation in monotectic alloys. Previous studies have focused on the phase structures, material properties, and thermodynamic behavior of Gd-Co-Ti ternary monotectic alloys. However, research on their solidification process is sparse. In this study, rapid and sub-rapid solidification experiments under drop-tube microgravity conditions were performed using Gd-Co-Ti ternary monotectic alloys. The effects of cooling rates on the solidification microstructure of the alloy were investigated. The resulting samples exhibited a composite microstructure comprising homogeneously dispersed subspherical TiCo-rich particles in the Gd matrix. These particles include: (i) TiCo-rich phase particles formed via liquid-liquid phase transformation, and (ii) TiCo-rich nanoparticles formed through desolventizing precipitation during the cooling process after solidification. To elucidate the microstructure evolution in Gd-Co-Ti alloys solidified under drop-tube conditions, a population dynamics model was established. The model comprehensively considers the thermal and mass transfer characteristics during solidification, as well as the nucleation, growth, and spatial motions of TiCo-rich phase droplets. The algorithm for solving the controlling equations in this model was developed based on the finite volume method. The microstructure formation was simulated, and the results were consistent with the experimental data, thus validating the accuracy of the model. The numerical results demonstrated that the nucleation of the TiCo-rich phase droplets occurred during the liquid-liquid phase transformation under drop-tube microgravity conditions. The number density of these TiCo-rich phase droplets remained unchanged after nucleation, indicating that the Ostwald coarsening of the TiCo-rich droplets was weak during the cooling of the alloy melt. Thus, nucleation and diffusion growth were the primary factors influencing the size of TiCo-rich phase droplets formed during the liquid-liquid phase transformation. With the increase in the sample sizes, the cooling rate of the alloy melt and the number density of the TiCo-rich particles decreased; thus, the average radius of the TiCo-rich particles in the solidification microstructure increased. Furthermore, the maximum nucleation rate (I_DMax) and the number density (N_D) of the TiCo-rich phase droplets/particles exhibited an exponential dependence on the cooling rate ({\dot{T}}_{\mathrm{n}\mathrm{u}\mathrm{c}}) during the nucleation period as per the following expression: I_{\mathrm{D}\mathrm{M}\mathrm{a}\mathrm{x}} = 7.202 × 10^-5{\left({\dot{T}}_{\mathrm{n}\mathrm{u}\mathrm{c}}\right)}^{\mathrm{2.2}} and N_{\mathrm{D}} = 3.385 × 10^-4{\left({\dot{T}}_{\mathrm{n}\mathrm{u}\mathrm{c}}\right)}^{\mathrm{1.3}}.