Classical molecular dynamics simulations are used to examine the effects of adsorption of inert-gas atoms (Ar, Xe, and He) at various pressures and temperatures on the shape, structure, and diffusion of a 256-atom platinum nanocluster supported on a graphite substrate. The many-body Sutton Chen potential is used to model the Pt-Pt interactions, and the Lennard-Jones potential is employed for the rest of the pair interactions. Gas adsorption on the metal cluster is monitored as a function of pressure at constant temperature, and the evolution of the structure is analyzed as the pressure is increased and then as the gas phase is gradually removed. It is found that the gas phase substantially alters the vacuum cluster structure, and the changes are mostly irreversible in the time frame of the simulations (2 ns), especially at temperatures well below the cluster melting point, where the low energy structures determined under high pressures are very stable. The dynamic properties are also modified by the gas phase; the cluster diffusion depends on the inert gas pressure, being lower at high gas pressures. Although the contact area cluster-substrate is significantly smaller compared to the vacuum case, the size mismatch is reduced because of the presence of the gas-adsorbed layer that forces the rearrangement of the metal atoms leading to distorted structures which favor the Pt-C interactions.