No, this is not true and you can’t do that. Please see this article: https://help.author.envato.com/hc/en-us/articles/360000534646 - where the GPL is explained clearly:
If an author is releasing a theme or plugin for a platform that is licensed under the GPL, the PHP code and integrated HTML of the theme or plugin must be distributed under the GPL.
So, when a customer purchases an item from ThemeForest or CodeCanyon, and finds the GPL license text enclosed, this license applies to the item’s PHP code and integrated HTML. Other assets that are a part of the item may not be covered by the GPL
Given that, this means that if you buy a 100% GPL WordPress theme/plugin, the PHP and the HTML code inside are GPL, while the other assets (images, JS and CSS code) are not GPL.
So the conclusion is that you can’t resell or redistribute for free the theme/plugin.