It is well-known that in elemental metals, the onsite Coulomb energy of transition-metal (TM) d-electrons, Udd, is significantly smaller than Uff of f-electron rare-earth (RE) metals. Consequently, Udd is often neglected in RE-TM intermetallic alloys. In spite of the low value of Udd compared to Uff, we quantify and clarify the important role of Udd in partially filled d-bands of RE-TM alloys. We investigate the electronic structure of a typical RE-TM ferrimagnetic series Gd6(Mn1−xMx)23 (M = Fe, Co; x=0.0,0.3), which shows promising magnetocaloric properties. Resonant photoemission and constant initial state spectroscopy is used to identify the Mn 3d, Fe 3d, and Co 3d partial density of states (PDOS) in the valence band. The photon energy-dependent spectral evolution allows us to separate out the lower Hubbard band and the two-hole correlation satellites in the Mn, Fe, and Co 3d PDOS. Using the Cini-Sawatzky method, we determine an average Udd=2.1±0.4eV, 2.2±0.4eV, and 2.9±0.4eV for the Mn 3d, Fe 3d, and Co 3d states, respectively. The relatively larger Udd for Co compared to Fe 3d states results in lower DOS for the coherent feature at the Fermi level (EF) and higher DOS in the lower Hubbard band away from EF in Gd6(Mn0.7Co0.3)23 compared to Gd6(Mn0.7Fe0.3)23. To understand the role of Coulomb correlations on the electronic structure and magnetic properties, ab initio electronic structure calculations using density functional theory with onsite Coulomb correlations (DFT+U) were carried out for the parent Gd6Mn23. The results show that the calculated Mn magnetic moments are consistent with experiments when UMnDFT=0.75eV, corresponding to Udd=1.65eV and Jdd=0.9eV. Further, using the calculated Gd and Mn PDOS and known photoionization cross-sections, the simulated Gd6Mn23 spectrum is fairly consistent with the experimental valence band spectrum. The results indicate the crucial role of d-d correlations in the presence of large f-f correlations for tuning the electronic structure and magnetic properties of RE-TM intermetallics.