Yes, I believe GM's stocks will tumble after they report earnings. Based on what I've read on the matter, GM's stocks aren't very lucrative anyway. They seem to tumble more than they rise. GM's stock plummeted the last time they reported their earnings so I believe that this time they will as well.
Yes, I believe GM's stocks will tumble after they report earnings as it has been a rough year economically and I believe most people are expecting all vehicle manufacturers stock prices to take a hit. When the economy is dropping, the number of people purchasing new vehicles also drops which has a direct impact on the value of their stocks.
Wall Street, on the whole, is about immediate gratification. GM has had to recall over 30 million vehicles this year and their stock has been down 25% from 2013. This is not a situation that looks immediately profitable, so short-sighted investors will dump GM's stocks once GM reports their earnings.
I think any company's stocks will fall a little after they report earnings. Investors always have a higher opinion on how much a company should be making and what they are doing with their money so it's only natural there will be some unhappy investors who will pull their money out even if they reported positive earnings.