It is possible to run a limited set of kdb commands from gdb, using the gdb monitor command. You don't want to execute any of the run control or breakpoint operations, because it can disrupt the state of the kernel debugger. You should be using gdb for breakpoints and run control operations if you have gdb connected. The more useful commands to run are things like lsmod, dmesg, ps or possibly some of the memory information commands. To see all the kdb commands you can run monitor help.

Example:

(gdb) monitor ps
1 idle process (state I) and
27 sleeping system daemon (state M) processes suppressed,
use 'ps A' to see all.
Task Addr       Pid   Parent [*] cpu State Thread     Command

0xc78291d0        1        0  0    0   S  0xc7829404  init
0xc7954150      942        1  0    0   S  0xc7954384  dropbear
0xc78789c0      944        1  0    0   S  0xc7878bf4  sh
(gdb)