Let's look at the OSI definition since that's what people mean by Open Source (particularly when capitalized):
> The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.
> No Discrimination Against Fields of Endeavor
You cannot restrict selling derivatives of the software (a field of endeaver), and the derived work must be able to be shared under a similar license.
GPL does not add any restrictions counter to that. It allows you to redistribute and sell copies if you want, you just have to respect your user's freedom by also giving them the modified source code.