Validation of GASTLI for Saturn and Neptune. Left: mass-radius relations for Saturn. GASTLI’s fiducial model for Saturn can reproduce Saturn’s mass and radius data with better precision than HG21. Here, we assume an equilibrium temperature of T_eq = 100 K across all models, while we adopt an internal temperature of T_int = 77 for Saturn. Models for Saturn were calculated for a fixed envelope metallicity of 8.9 × solar, equivalent to Z_env = 0.10 for the HG21 model. Right: mass-radius relations for Neptune. The fiducial model (black) agrees well with mass and radius data. Here, we assume an internal temperature of T_int = 52 K for Neptune.