This study uses the Moore-Gibson-Thompson (MGT) theory of modified coupled stress to examine the combined effects of gravity and an internal heat source on a thermoelastic medium. The model integrates gravity as a body force and an internal heat source to account for microscale stress interactions and finite-speed thermal wave propagation, factors often missing in standard models. The governing equations are solved using the normal mode approach, and MATLAB is used for analysis. Results are compared across different scenarios in both the MGT and Green-Naghdi III (GN III) frameworks, considering variations with and without gravity, internal heat sources, and couple stress parameter. These findings enhance predictive modeling for high-temperature applications, such as aerospace and geophysics, by offering insights into the durability and performance of thermoelastic materials in complex conditions.