> I tried M-x gdb to run emacs under gdb.  The command line was
>
>   gdb -i=mi ~/src/emacs/src/emacs
>
> and I got the error '(json-object-format ":" 44).  Here is the backtrace:

Thank you for reporting this bug.

I could reproduce your problem and I wrote a workaround which fixes the
bug for me. Could you please try the attached patch for gdb-mi.el?