Hybrid heterostructures, made of organic molecules adsorbed on two-dimensional metal monochalcogenide, generally unveil interfacial effects that improve the electronic properties of the single constitutive layers. Here, we investigate the interfacial electronic characteristics of the F4-TCNQ/single-layer GaSe heterostructure. A sharp F4-TCNQ/GaSe interface has been obtained and characterized by x-ray photoemission spectroscopy. We demonstrate that a high electron transfer from 1TL GaSe into the adsorbed F4-TCNQ molecules takes place, thereby yielding a reduction in the excess negative charge density of GaSe. Additionally, the direct band-structure determination of the heterostructure has been carried out using angle-resolved photoemission spectroscopy, shedding light on essential features such as doping and band offset at the interface. Our results indicate that the buried 1TL GaSe below the F4-TCNQ layer exhibits a robust inversion of the valence dispersion at the Γ point, forming a Mexican-hat-shaped dispersion with 120 ± 10 meV of depth. Our experiments also reveal that F4-TCNQ can significantly tune the electronic properties of 1TL GaSe by shifting the band offset of about 0.16 eV toward lower binding energies with respect to the Fermi level, which is a key feature for envisioning its applications in nanoelectronics.