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I would agree with DonXoop here - you are the author and can do what you want with the code, since it must be GNU/GPL licensed. However, if you are an employee in a company (even a temp) I would think that what you do in your working time for them, they can use, too.
So basically, you can give out or sell the code to anyone you want, but you cannot restrict its use to the company, you made it for or the people, who buy it (in plain terms: Since you spent your working time doing this for the company, they have a right to use it. They cannot prevent you from selling the code or giving it away to other people, and you cannot prevent those people from giving it to someone else).