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The made up a case number and when I called by he wouldn't tell me why I was called excecpt that GMC filed on me. I called GMC directly and they said it was a fake and they didn't know who I was...which they shouldn't have since I haven't had anything with GMC

So you knew you never dealt with GMC before and called them to confirm you were right?? Wouldn't knowing you never dealt with GMC be enough to convince you in the first place that it was a debt scammer or are you telling that story to convince others here?

On top of what Mike said you even decided to do their work for them.  Are you going to get paid for that?

It is incumbent upon them under the law to prove that the debt exists and that you owe it, and (this is the important part) that they have the legal right to collect it. You are not obligated under the law to prove that you don’t owe or that it is paid.

Federal law (FDCPA) requires them to send you a letter, email, or text (postmarked in the case of a letter) within 5 days of their first contact that contains their name, physical address, the creditor’s name, and the amount of the alleged debt. Unless they have communicated it to you verbally at the beginning of your conversation. It also must contain the “mini-Miranda” telling you that it is an attempt to collect a debt and that all information will be used for those purposes. The one other important thing that this communication must also have in it is that you have a right to dispute the debt within 30 days of receipt of the letter and if you do so, all collection activity must be stopped until the debt is verified. If and when you get that communication you should immediately send that debt validation letter by certified, return receipt mail.

First, you should make a complaint at this Federal Agency, and while there you should also read up on how debt collection is supposed to work as well as what your rights in this matter are: https://www.consumerfinance.gov/

Also file a complaint with your State Attorney General's office.
List of State AG’s offices: https://800notes.com/faq/attorney-general