I know the debugger gem is not and never will be compatible with ruby 2.0 per "officially support ruby 2.X".
In the changelog of Ruby 2.0 is:
DTrace support, which enables run-time diagnosis in
production TracePoint, which is an improved tracing API
The debugger gem can be used but it still has issues.
Install byebug which was written for Ruby 2.0 debugging.
For breakpoints, use the
byebug command in your code instead of