No, this would not be grounds for dismissal. As a felon you are not allowed to own, carry or transport any weapon. Your girlfriend may have put it in your vehicle without your knowledge, but you knew it was there when you got in your car and you said as much to the police. That gave the police probable cause to arrest you for the weapon's possession.

If you go to trial you may be able to establish temporary lawful possession if a jury believes that you were on your way to take the gun back to your girlfriend when you got stopped by the police. You do have a signed statement that the gun was placed in your car without your knowledge. And there may even be constitutional issues about the statement you gave to the police telling them about the gun and their subsequent search without your permission which could be explored in a pre-trial hearing where, if your lawyer was successful at the hearing, the gun could be supressed

But that's way down the road. Meanwhile, you will have to fight the charge, as I don't see a dismissal under these fact patterns.

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This thread will not close and you can always use it to get clarification.This is informational only and is NOT legal advice.