In order to investigate the role of gravity in a novel cancer treatment strategy called Gas Embolotherapy, we have computationally studied the evolution dynamics of two bubbles sticking to and sliding on the opposite walls of a 2D channel, under gravity-driven flow. We have modeled the moving three-phase contact lines using Tanner laws including contact angle hysteresis and have accounted for the gas-liquid interfacial dynamics in our model. Our model uses a Boundary Element Method (BEM) based moving-interface, multi-domain, iterative method to compute the flows and stresses on the domain boundaries at various instants of time. Since the normal and shear stresses acting on the endothelial layer of blood vessels are a major concern in the development of gas embolotherapy, we have examined the effect of bubble evolution and induced flows on the wall stresses. For a range of initial bubble pressures, we have studied the role of gravity by varying the Bond number and by using two different inclinations of the channel (horizontal and vertical) with respect to gravity. Our results suggest that the strength of gravitational forces and the inclination of the channel have a pronounced effect on both the bubble evolution and the resulting wall stresses. Aside from gravitational effects, the interaction of the bubbles through the surrounding fluid has a significant effect on their evolution. We have also examined the flow rates at both ends of the channel resulting from the evolution of the two bubbles.