It's complicated.
As others have already stated, "distributing" (GPLv2) or "conveying" (GPLv3) software by selling it is explicitly permitted by both the GPLv2 and GPLv3 as long as the terms of the license are not violated. Any licensee is completely free to sell GPL software and make money off of it.
The complication stems from the additional legal and technical restrictions app stores tend to impose. As a particularly infamous example, Apple purged their app store of all GPL-licensed software in the early 10s because they were found to violate the terms and conditions of the GPLv2. As the FSF put it:
The primary problem is that Apple imposes numerous legal restrictions on use and distribution of GNU Go through the iTunes Store Terms of Service, which is forbidden by section 6 of GPLv2.
At the time, Microsoft also banned all GPL software from their store (as reported by arstechnica):
"Excluded License" means any license requiring, as a condition of use, modification and/or distribution of the software subject to the license, that the software or other software combined and/or distributed with it be (i) disclosed or distributed in source code form; (ii) licensed for the purpose of making derivative works; or (iii) redistributable at no charge. Excluded Licenses include, but are not limited to the GPLv3 Licenses. For the purpose of this definition, "GPLv3 Licenses" means the GNU General Public License version 3, the GNU Affero General Public License version 3, the GNU Lesser General Public License version 3, and any equivalents to the foregoing.
Since the early 10sFortunately, the situation has improved drastically since the early 10s. Both companies have revised their EULAEULAs to be more compatible with the GPL. Apple for example has emended their EULA to allow sellers to override their standard EULA:
Any App that you acquire is governed by the Licensed Application End User License Agreement (“Standard EULA”) set forth below, unless Apple or the App Provider provides an overriding custom license agreement (“Custom EULA”).
Microsoft has done something similar with their EULA:
ENTIRE AGREEMENT. This agreement, any applicable privacy policy, any additional terms that accompany the application, and the terms for supplements and updates are the entire license agreement between you and application publisher for the application.
From all of this, a person can reasonably conclude that anyone is allowed to resell GPL software on Microsoft's store AND Microsoft is allowed to distribute said software as long as the end user can fully exercise their rights. Generally, this holds true on x86-64 platforms which tend to be open platforms. There might be legal issues with selling GPL-licensed software for the XBOX platform and ARM-based systems which tend to be locked down by Microsoft via DRM and thus completely running afoul of section 10 paragraph 3 of the GPLv3.
Last year, I actually emailed Mr. Stallman, the primary author of the GPLv3, asking him about his opinion on this specific issue. He seemed to believe that the way App Stores currently distribute GPLv3 software to locked devices violates the terms of the license.
In summary:
- Selling GIMP for x86-64 Windows is probably okay.
- UnclearIt is still unclear if okaythe seller is allowed to sell GIMP for XBOX, ARM-PC or any other device where Microsoft locks down the device or requires itthe device be locked by OEMs.
Disclaimer: I'm not a lawyer. This is not legal advice. YMMV.