> Basically, start GDB with Emacs as the executable to debug, then run
   > Emacs from GDB's command line.


Oh well,

I tried gdb /opt/emacs27/bin/emacs
and I obtained
Reading symbols from /opt/emacs27/bin/emacs...(no debugging symbols
found)...done.

But no emacs started.

The message indicates that I should recompile emacs?

   > No need to google, the answer is under your fingertips: see the
   > beginning of the file etc/DEBUG in the Emacs source tree.

Thanks, but just read it, but it is a bit confusing, since it is not
clear to me whether I could start emacs first and then start gdb within
emacs.

So it comes down to this: is 

gdb /opt/emacs27/bin/emacs

in the emacs/src directory the right command to use?
I tried it but emacs does not start, what do I miss?


So shall I recompile emacs with debug options like

  CFLAGS='-O0 -g3' ./configure --enable-checking='yes,glyphs' --enable-check-lisp-object-type