Is it that.. you find it tricky to get in place a particular sequence of C FFI structure release function invocation on their GC???

It's not tricky if you know what you're doing. But if you are going to publish a library that exposes C bindings, you really don't want to have to be warning people against holding weak references to these foreign objects, lest they get a segfault or memory corruption.

Something along the lines that if you have a C structure A that contains in it a reference to another C structure B, and you want Gambit's GC to automatically take care of this relationship in the sense that it will not release B before A??

Yes, with the nuance that the C structure B is contained in A, not just referenced from it. So it doesn't make sense to free B at all. Another use case is that A is a union and B is another pointer to A's data. You want to free A only once.

But yes, the main point is that you want A to stick around while you have a reference to B. This kind of sanity you take for granted in the Scheme world, thanks to garbage collection. What I want is to be able to treat C objects similarly, without having to do unnecessary copies.