I would make an initial attempt to remove the lien without the help of an attorney, so you do not incur attorney fees. Make sure to keep record of all contacts with GM Financial and the result of those contacts. You will likely need to show proof that the loan was paid in full.
If you cannot resolve the issue with GM Financial you may have a spurious lien case. In Colorado, if you believe that a spurious lien or document has been filed against your property, you can petition the district court or the federal district court in the state for an order to show cause.
This petition should detail why the lien or document is invalid and must be supported by an affidavit. Following a hearing, if the court determines the lien is indeed spurious, it will invalidate the lien and award you costs, including attorney fees. However, if the court finds the lien to be legitimate, you will be required to cover the respondent's costs, including attorney fees. In my opinion, you would need the assistance of an attorney to prosecute this type of case. For more information see C.R.S. § 38-35-204
It's important to resolve this as soon as possible so that GM Financial does not initiate collection actions that could impact your credit or in the worse case attempt to repossess the car.