Say you want to try running file system server (ext2fs, ?jfs, ...) against a modified version of libpager and debug the latter one using GDB.

Set the translator like this:

$ settrans -fgap ↩
  jfs.img.i ↩
  /usr/bin/env ↩
    LD_LIBRARY_PATH=~tschwinge/tmp/hurd/hurd.ogi/build/libpager ↩
  "$PWD"/jfs.obj/jfs ↩
    "$PWD"/jfs.img

Then, when starting GDB, start it like this (or use an equivalent method, of course):

$ LD_LIBRARY_PATH=~tschwinge/tmp/hurd/hurd.ogi/build/libpager ↩
  gdb ↩
    ~tschwinge/tmp/jfs/jfs.obj/jfs 15436
GNU gdb 6.5-debian
[...]
Reading symbols from /lib/libdiskfs.so.0.3...Reading symbols from /lib/debug/lib/libdiskfs.so.0.3...done.
done.
Loaded symbols for /lib/libdiskfs.so.0.3
Reading symbols from /devel4/tschwinge/tmp/hurd/hurd.ogi/build/libpager/libpager.so.0.3...done.
Loaded symbols for /devel4/tschwinge/tmp/hurd/hurd.ogi/build/libpager/libpager.so.0.3
Reading symbols from /lib/libstore.so.0.3...Reading symbols from /lib/debug/lib/libstore.so.0.3...done.
done.
Loaded symbols for /lib/libstore.so.0.3
[...]
(gdb) break pager_demuxer
Breakpoint 1 at 0x105c98d: file ../../libpager/demuxer.c, line 27.
[...]
Breakpoint 1, pager_demuxer (inp=0x12bded0, outp=0x12bbec0) at ../../libpager/demuxer.c:27
27      {
(gdb) list
22      /* Demultiplex a single message directed at a pager port; INP is the
23         message received; fill OUTP with the reply.  */
24      int
25      pager_demuxer (mach_msg_header_t *inp,
[...]

Voilà.