Meroon is a Scheme object system by Christian Queinnec. It is portable, fast, CLOS-like and reflexive. The Meroon home page cites versions for several popular Scheme systems, including Gambit-C. But the links to the Gambit-C version are broken. Doing a bit of research, that actually involved some guessing, I found the Meroon Gambit-C port in Bradley Lucier’s home page. Instructions to build this Meroon port can be found in this thread in the Gambit-C mailing list.
I am not a heavy user of object orientation, but I can certainly see its value where it is worth. In the LiSP book, for instance, a beautiful interpreter and a compiler are written in OO-style, in chapter 3 and 10, respectively. Another example is writing a scene graph for a Computer Graphics application, it almost begs to be written object-oriented.