How to undo codesign?

It's not well documented at all, from what I can tell, but you may be able to remove code signing via:

codesign --remove-signature gdb

I had a problem with removing signature as suggested by Micheal:

unsupported type or version of signature

Finally I used -f flag and force loaded new certificate.

Add --force to the arguments of your codesign command to make it re-sign something.

For example:

codesign --force --sign gdb-cert gdb