Heather_dw,
Ask for debt validation. Also inform them that you would prefer to be contacted through mails. You may contact them at the GC Services address or may call them at 877-290-8894.
Always use CMRRR while corresponding. It will give you immense support to defend their unfair claims in future.
If they continue calling you after receiving your letter, you can report against them with FTC, BBB or attorney general's office of your state.
If you have any trouble in repaying the student loans, try to consolidate it. Student loan consolidation opens a treasure of facilities for the debtors. Review the benefits of
student loan consolidation.
Feel free to discuss your problems in forums.