Most of the available commercial solid-state gas/vapor sensors are based on metal oxide semiconductors. Metal oxides (MOs) change their conductivity while exposed to gas or vapors ambient can be utilized as gas or vapor sensing materials. In recent days, graphene has attracted tremendous attention owing to its twodimensional structure with an extremely high surface to volume ratio, electron mobility, and thermal conductivity. However, intrinsic graphene is relatively inefficient for the adsorption of gas/vapor molecules. In this regard, graphene oxide (GO) and reduced graphene oxide (rGO), which are graphene species functionalized with different oxygen groups that offer a higher amount of adsorption sites improving the sensitivity of the film. Up to now, many research groups across the globe have reported the promising performance towards gas detection using various GO/rGO-metal oxide nanocomposites. This chapter reviews the composites of graphene oxide or reduced graphene oxide and metal oxides in nanoscale dimensions (0-D, 1-D, 2-D, and 3-D) for gas sensing applications considering two specific focus areas, that is, synthesis of nanocomposites and performance assessment for gas/vapor sensing.