They are making threats that could be determined to be in violation of the FDCPA.
They will have to sue you first, obtain a judgment, and then they would be able to garnish wages if allowed in your state.
Here is a link to your rights.
Unfortunately, your failure to purchase gap insurance has left you in a bad position. Should they decide to take you to court,
the chances that they will prevail are very good.
http://www.ftc.gov/bcp/edu/pubs/cons...edit/cre27.pdf