Hello. I see that your project is licensed under the GNU GPL 3.0. Do you permit this project to be licensed by any newer versions of GNU GPL that may be released in the future?

This is related to section 14 of the GNU GPL 3.0 license:

14. Revised Versions of this License.

The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.
Each version is given a distinguishing version number. If the Program specifies that a certain numbered version of the GNU General Public License "or any later version" applies to it, you have the option of following the terms and conditions either of that numbered version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of the GNU General Public License, you may choose any version ever published by the Free Software Foundation.

If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Program.

Later license versions may give you additional or different permissions. However, no additional obligations are imposed on any author or copyright holder as a result of your choosing to follow a later version.

Projects usually differentiate this intent by using a different license header notice which is put into all code files. If only 3.0 is permitted, the following license header is used:

<one line to give the program's name and a brief idea of what it does.>
Copyright (C) <year> <name of author>
This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 3.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see <https://www.gnu.org/licenses/\>.

If 3.0 or any other version is used, the following header is used:

<one line to give the program's name and a brief idea of what it does.>
Copyright (C) <year> <name of author>

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see <https://www.gnu.org/licenses/\>.

A quick look at the codebase showed that it doesn't add license headers to code files (please correct me if I'm wrong). It also doesn't explicitly state which license is meant to be used in the README or any other documentation source.

By neglecting to state whether later versions are permitted, (I assume) newer versions are implicitly not allowed (matching GPL-3.0-only SPDX license identifier). But stating this more explicitly would help packagers like me.

I recommend you to add a small section to the README saying that the project is licensed under either GPL-3.0-only or GPL-3.0-or-later, identifying the exact license by its SPDX license identifier.