I've noticed that when a thread is asleep and (gc) or scm_gc() is called, it gets awoken. It can be demonstrated with the following example:

> (call-with-new-thread 

    (lambda()

      (while #t 

        (display "tick!\n")

        (sleep 5)))) ;; <= this is 5 seconds!

> (gc)

tick! ;; this happens immediately after each call to gc!

This behaviour is both unexpected and undocumented, so I consider it a bug.