Michael points out:

...

Just I've tried by adding "-comspeed 9600" option to rmgr, also ^^^^ "-comspeed 9600" is not an Rmgr option but an option for the Fiasco kernel, like this: (slightly edited from my menu.lst)

I see! Adding the option to the Fiasco kernel, GDB successfully connects to the target PC. Sooner I will check the revised Fiasco. I was reading the manpage of L4 (telling "-comspeed" as the option for rmgr) and not of Fiasco (telling it as for main). I was wondering why rmgr does not understand options like "-comspeed", "-comport".