The RS CVn type binary star GT Muscae was observed during its quiescence using the Resolve X-ray microcalorimeter spectrometer onboard XRISM. The main and satellite lines of the Fe XXIV–XXIV K-shell transitions were resolved for the first time from stellar sources. We conducted line ratio analysis to investigate any deviations from collisional ionization equilibrium and Maxwell electron energy distribution with a single temperature. By using five combinations of direct excitation lines and dielectronic recombination satellite lines in three line complexes (Fe He$alpha$, Ly$alpha$, and He$beta$), we found that the plasma is well characterized by two-temperature thermal plasmas with temperatures of 1.7 and 4.3 keV, which is consistent with thermal broadening of Fe XXV, and the broad-band fitting results in the 1.7–10 keV band. Other forms of deviation from a single-temperature plasma, such as different ionization and electron temperatures or the $kappa$ distribution for the electron energy distributions, are not favored, which is reasonable for stellar coronae at quiescence. This study demonstrates the utility of the Fe K-shell line ratio diagnostics to probe plasma conditions using X-ray microcalorimeters.
