If I own stock in a company that sold to another, and now we own shares in the new company, are the gains reportable?

[JamesG1]

If you did not sell any shares of stock, your cost basis does not change, the same cost basis is now allocated over all the shares of stock.  Here is an example.

If you bought 11 shares of stock A for $110, your cost basis in that investment is $110.

Stock A spins off another corporate entity on the basis of 1 share of stock B for every 11 shares of stock A.

Now you have:   

Stock A        11 shares    $100.83    1/12 X $110 = $.083 X 11 shares

Stock B        1 share        $9.17        1/12 X $110 = $.083 X 1 shares

12 shares    $110

Your basis still remains $110 for the total investment.

[TomYoung]

If you got a 1099-B then you have a sake that you need to report; the IRS will expect to see it on your income tax return.  Since you said you have a gain I'd expect you did get a 1099-B.