In Debian, to get ?debugging information for glibc, you need to install the libc0.3-dbg package. At the place ?GDB looks for debugging symbols by default (/usr/lib/debug/.build-id), Debian's libc0.3-dbg stores only the frame unwind information used for backtracing.

You also need to download the source code with apt source glibc and cd into it for gdb to be able to find and show it, or use set directories inside gdb, or put it in your .gdbinit