That's correct. Anything you can debug with gdb should be debuggable under rr. (Though of course there could always be rr bugs that need to be fixed.)
Languages where debugging requires complex VM services are another story. For example, if your language supports debugging by modifying the code generated by a JIT, that's going to be a problem under rr, since an rr replay preserves the exact instructions executed during recording.
Debugging a non-trivial application with gdb without gdb crashing is quite a feat, however. Hopefully rr is less prone to segfaulting. Gdb is easily the least stable standard debugger I've used on any preemptive scheduling OS.
