We present new empirical calibrations of the electron temperature–metallicity relation in Galactic H II regions using combined radio and optical observations from the DEep Spectra of Ionized REgions Database (DESIRED) project. Our analysis is based on over 200 deep optical spectra with [N II] and [O III] temperature diagnostics, and more than 450 radio measurements of the global electron temperature across star-forming regions in the Milky Way. This multi-wavelength approach allows us to derive robust oxygen abundance gradients from the inner to the outer Galaxy (0.1–16 kpc). We compare these nebular metallicities with independent measurements from young O- and B-type stars and classical Cepheids. Temperature-metallicity calibrations that include internal temperature fluctuations in the ionized gas show excellent agreement with stellar tracers, while those assuming uniform temperatures underestimate abundances by up to 0.3 dex. By reconciling nebular and stellar metallicity gradients through physically motivated temperature calibrations, this work provides observational support for the presence of thermal inhomogeneities in ionized gas. In addition, the lack of significant azimuthal metallicity variations (greater than 0.1 dex) suggests efficient mixing processes across the Galactic disk. Together, these results establish a more reliable framework for tracing the chemical evolution of the Milky Way.