When applying the GPL, you must choose whether to limit it to just that version of the GPL, or whether to allow any future versions also. Generally this is done in the license header in the source files; however, the source files for qFlipper do not have a license header in them, so it is currently ambiguous whether qFlipper is licensed GPL-3.0-only or GPL-3.0-or-later. I would recommend either adding SPDX License IDs to the headers of the source files, or at least adding a line to the README that specifically states which variant is in use.
When applying the GPL, you must choose whether to limit it to just that version of the GPL, or whether to allow any future versions also. Generally this is done in the license header in the source files; however, the source files for qFlipper do not have a license header in them, so it is currently ambiguous whether qFlipper is licensed GPL-3.0-only or GPL-3.0-or-later. I would recommend either adding SPDX License IDs to the headers of the source files, or at least adding a line to the README that specifically states which variant is in use.