> sorry, I mean "how do I set dpb_garbage_collect" because I suspect
> that the default is dpb_no_garbage_collect...

There is no "dpb_garbage_collect" flag. Garbage collection is always on by
default. You can try isc_dpb_sweet to start the database sweep, but I am not
sure if that is correct approach.

If you have more questions to GC in Firebird, please use Firebird-Support
list.

Roman