I've often missed turning on tracing on the gc only. As it is now, one can get tracing from the gc on trace level 1, but that at the same time turns on tracing on a multitude of other things which makes it mostly useless.
This can either be fixed by adding a separate trace flag for the gc and some accompanying gc_trace() function, or by increasing all the current trace levels to make room for a new trace level 1 that only traces the gc. I think it'd be more consistent with the current trace system to do the latter. What do you say?