rb_gc_mark prevents the value from being garbage collected if called outside of a GC cycle
I had some code that called rb_gc_mark in an initializer function instead of in the corresponding mark function, and the value never got garbage collected, causing a memory leak. This seems like an easy mistake to make, and there does not appear to be a corresponding function to make the value eligible for garbage collection again.
If the intention of the API when calling it in that situation is to make it a root object, then there should be a function to remove that indicator once the object no longer needs to be live. And if the intention is for the function to only work properly during a GC mark phase, then it should be an error to call it at any other time.