There is open-source project under GPL 3 hosted on GitHub. The project contains source code and assets (images, sounds etc.). Can I legally use only assets (without soruce code) for custom, commercial closed-source project?
The GPL is focussed on software, so it is not clear how it applies to non-software creative works such as images. But unless the license allows you to do something with these assets, you cannot use them – they are still copyrighted works!
I interpret the GPLv3 so that you can use these assets in a proprietary project. However:
If you use the unmodified assets you still have to provide the GPLv3 license terms to end users and must not prevent them from copying and modifying these assets.
If you modify the assets then these modified assets would also be subject to the GPLv3. You not only have to mention that you modified them, but also have to provide the preferred form of modifying these assets. E.g. if you modify an image with Photoshop, you would have to provide the .psd file.
The GPL would extend to anything that is a derived work of these assets. What falls under this is difficult to tell, especially for audiovisual works like video games. Because of significant legal uncertainty, it might be better to avoid these assets.
You can of course always contact the authors of these assets and ask them for a more suitable license.
GPL v3 defines the “The Program” as "any copyrightable work licensed under this License". That would include works which are literary works (the most common form of a computer program), and artistic works (ie images), sound recordings and films.
Artistic works and sound recordings would also be a "covered work", and if you edited the image, the edited work would be caught by the definition of "modify".
By slotting photographs (ie artistic works), sounding recordings and films into a GPL 3 licensed package, the starting point is that its use is covered by the licence. I think it's a hard one to dodge.