The base wp-e-commerce plugin is free (as in both beer and speech). In other words there is no charge, and no restrictions on its use (except as specified by the GPL).
The gold cart plugin is a paid-for add-on providing extra features. It is released under the GPL but the developers charge a (small!) fee to download it which is their right under the GPL.
This may help:
https://ivycat.com/a-newbies-guide-to-the-gpl-and-wordpress-licensing/
All wordpress plugins should be distributed under the GPL: if you pay for them then you are generally paying for support and updates.