sd386 looks nice. Are there any tutorials on its various features? It has source code. Has it been ported to any other OSes, so the OS/2 guest being debugged can be debugged from a Win32 or DOS or Linux host?
Not that I know off and considering it was replaced by graphical debuggers that likely ran on Windows, AIX and perhaps other systems, probably not, especially as the readme infers the remote debugging support was added just before being discontinued.
With source code available, a stub could probably be created, and the actual debugger app could be ported to another OS.
I never used to admire remote debugging, but I've had quite a bit of occasion the past couple years to do remote debugging over TCP/IP. It's quite nice on a fast LAN. And it's invaluable to be able to reach out to a client's system and diagnose their app errors from the comfort of my office, rather than driving to other cities. :-)
Note that to use any of the IBM debuggers with GCC, programs need to be linked with -Zomf as well as built with -g
I've had to use GCC when I've had to, but I've found GCC and GDB to be inferior debugging tools. It's why I've done most of my development in Win32. Visual Studio 2019 Community is free for up to 5 developers to use for proprietary / commercial work. If you're an open source developer, it's free for any number whether proprietary or not. It has features like edit-and-continue debugging, where you can change your source code while the program is running, apply code changes, and keep going.
There's no other development environment like it on the planet. If there is, I'd like to see it.