I hava a Cybex Mios2, can I use the Mios seat on the Cybex Gazelle S chassis?
No, you can't, I am sorry. The connectors of the Cybex Mios are different than the Gazelle S's, so you won't fit parts of one to the parts of others, it's not possible.
P.S. Even if you could, the frame of the Gazelle is counting on a certain center of gravity, and the smaller, lighter Cybex Mios's seat unit might make the push and the stability comprmised. Just FYI, it's good to know. In any case, the Gazelle frame and the Mios seat are not compatible.