But gdb probably will point to C source line instead of nim's source. That might be a problem.
No, you get Nim source code debugging using line number directives etc, works great including breakpoints and all. I have tested several IDEs and it works. But data inspection does generally not work - that would require some work to support GDB protocols.
Fortunately, GDB is scriptable in Python. The C++ add-ons make life so much easier, no reason it can't be done for Nim.