Currently Buildship does not provide debugging support. We have plans to add this feature, but I nothing is scheduled yet.
According Gretty’s documentation, it is possible to enable debugging for the servlet container:
http://akhikhl.github.io/gretty-doc/Debugger-support.html
With that, you can try using Eclipse’s remote debugging functionality:
http://help.eclipse.org/mars/index.jsp?topic=%2Forg.eclipse.jdt.doc.user%2Ftasks%2Ftask-remotejava_launch_config.htm