You can enter R&D mode without flashing, but it will not help, as gainroot depends on sudo to gain root access. If you did not prepare in advance (sshd, another boot partition, some suid root binary that can run a shell), you are out of luck.

There are many programs that run as root on the system. If you can find a vulnerability in them that enables you to run arbitrary code, you might be able to fix the problem without a reflash.