This study investigated the physical properties of Gmelina arborea wood following thermal modification using the open and closed processes. The aim of examining these processes on gmelina wood is to find a suitable processing condition that will offer acceptable reduction of moisture ingress, deliver good quality of thermally modified (TM) wood, while enhancing other important technological properties based on target end uses. For this study, thermal treatment using the open process covered three temperature regimes; 180 °C, 200 °C, and 220 °C. Modification in the closed process was done under temperature regimes including 160 °C, 170 °C and 180 °C, and under low pressure, as use of high pressure have been reported to cause more severe degradation to wood polymers. The outcome of the above modification methods showed that corrected mass loss of samples modified under both processes increased with increasing temperature; but higher losses was recorded in the open process. Mass loss significantly influenced the final moisture content, and slightly density through the open process. Volumetric swelling was also reduced from 9.86% to 3.8% for samples modified at 220 °C in the open process, while similar volumetric swelling of 5.41% and 5.26% was recorded for samples modified at 180 °C in open and low-pressure closed process respectively. From these results, gmelina wood showed improved resistance to moisture ingress in the open process at higher temperatures between 200 °C to 220 °C compared to the low-pressure closed process. However, thermal modification of gmelina in the open process negatively influenced its final quality and may have further implications for its strength properties.