Thanks to Andrew Stillman and his crew, you can now use Goobric with assignments created in Google Classroom. Doctopus is still a very useful tool and has features that are not available in Classroom yet but they are working hard to make Doctopus work with Classroom. Goobric allows you to use rubrics to assess and provide feedback to students with the rubrics being inserted into the document while the scores are recorded in the spreadsheet.
Watch the video below by @MsMagiera to see how to get started. You can also find new information in a blog post by Andrew Stillman at https://plus.google.com/+AndrewStillman/posts/VtzTvLzMWbW