First off all even if GM files for bankruptcy,(which they won't) that doesn't mean they will disappear. Hundreds of companies continue to operate under bankruptcy and most of them emerge from bankruptcy a better stronger company. Bankruptcy does not automatically mean you shut the doors and close down. In fact it rarely means that. What it means is you are given time to re-organize, re-group and re-build and then re-invent yourself.
That is like saying when a person files for personal bankruptcy that they are never allowed to work or own anything ever again.
Now, back to your question. No, bankruptcy does not mean your car will gain value. Your cars rarity and condition is what dictates it's value.