This issue started with:
[RFA] Fix GDB's handling of the inferior controlling terminal.
http://sourceware.org/ml/gdb-patches/2008-03/msg00103.html
(as reported by Alan Matsuoka)

The changed fixed other problems described there.

Either you should ignore the warning as it has the same functionality as gdb-6.x
before the change above.  For example signals do not work in that TTY, though. 
If you want to benefit from the fully dedicated TTY to the inferior process you
should run first the helper pasted below.

(One can discuss whether to call TIOCSCTTY with 0 (current) or 1.  IMO the
current value is right, 1 would not make it much easier and it would just
require root privileges plus make it whole less clear.)

I would suggest for GDB to make some notice to such a helper in the warning.
Thanks for the bugreport, is it OK this way?

------------------------------------------------------------------------------

/* tty;exec disowntty  */
#include <sys/ioctl.h>
#include <unistd.h>
#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
#include <signal.h>
static void
end (const char *msg)
{
  perror (msg);
  for (;;)
    pause ();
}
int
main (void)
{
  void (*orig) (int signo);
  setbuf (stdout, NULL);
  orig = signal (SIGHUP, SIG_IGN);
  if (orig != SIG_DFL)
    end ("signal (SIGHUP)");
  /* Verify we are the sole owner of the tty.  */
  if (ioctl (STDIN_FILENO, TIOCSCTTY, 0) != 0)
    end ("TIOCSCTTY");
  /* Disown the tty.  */
  if (ioctl (STDIN_FILENO, TIOCNOTTY) != 0)
    end ("TIOCNOTTY");
  end ("OK, disowned");
  return 1;
}