In this episode, Amy and Susan share tips on handling offers for internships and jobs in government, policy, advocacy, nonprofits, and international affairs. Some resources you might consider using:

Office of Personnel Management (OPM) General Schedule (GS 7 is the rate most undergraduates will enter at while graduate students will enter at GS 9 for MA/MS/Law and GS 11 for PhD).
WashU Career Center's handout on Negotiation - Not specific to these fields, but a great introduction.
National Association of Colleges and Employers Salary Survey: Note that this is not specific to GPP fields.
Pew Research: Facts about Pay Gaps
US Census QWI Explorer