Is there a way for gambit to generate a surface from two contours, say, a circle on one side and a ellipsoid on the other?

just with those 2 curves, I would say no.

But you can split each edge (at the middle for example), and create curves to join the two top surfaces.

Then create the lateral surfaces.

thanks for you response. My opposing contour is actually more complicated than an ellipsoid. It is a rectangle with two half circles at the ends. What you suggested is what I was thinking to do but only with more splits.

yes you can try. More split will accurate the surface