Specify the license of the project accurately
Currently the project root, and some other folder carry the GPL-3.0 license. However, as per the REUSE project (https://reuse.software) we should specify whether the accurate license description is:
GPL-3.0-only
or
GPL-3.0-or-later.
The above are SPDX (https://spdx.org) compliant ways to refer to the GPL-3.0 license. For those who do not know, the difference is that the latter license expression allows the licensee to choose eventual later versions of the GPL-3.0 license, whereas the first one is limited to the version 3.0 only.
Personally I would suggest GPL-3.0-or-later for this project, as it allows better compatibility going forward in the years.
(I have a couple of other related license practices issues, which I will return to once this is solved. These are adding SPDX compliant license statements to each file in accordance with the REUSE project. And making clear that the project works under the principle of inbound license equals outbound license. These related issues are reasonably separate and depend on the way this issue is solved, hence I will return to them later.)