The single opening makes these processes happen concurrently. Gastrovascular cavity circulates water through the animal which helps bring the nutrients to the body walls where cells pick up what they need from the water. Excretion occurs at the cell level into water. Gas exchange and circulation in Cnidaria occurs by diffusion. Since no cell is very far from the water environment, gasses and other nutrients can diffuse relatively easily from the environment to all cells.
Diffusion occurs based on differences in concentration.
I think they respire by osmosis. As far as circulation, the organism probably pushes water in and out through its movements which causes a flow of water in and out of the gastrovascular cavity. Gas is exchanged through simple diffusion. Likewise nutrients also are circulated by simple diffusion into each cell. Water movement is aided by the gastrovascular cavity.