@romu, @disho, @christian.krammer, @X-Raym, @marcosdourado.
I will explain why it does not work
In the desktop app, Deepin File Manager and Dolphin are not GTK-based, so you can not drag and drop. Only in the web app, you can. Deepin is QT-based and Dolphin is KDE-based. Therefore, Gravit Designer is GTK-based.
You can not drag and drop any file from non-GTK-based app on any GTK-based app. In spite of it, Atom, Chrome, Firefox and VSCode are GTK-based apps and accept receiving files from non-GTK-based apps.
If Firefox and VSCode were KDE-based apps, they will show the Dolphin dialog window instead of X11 dialog window, which is GTK-based. If Gravit Designer was QT/KDE-based, you would be able of dragging and drooping any files on it from a QT/KDE-based file manager and it also would show Dolphin dialog window.
In both desktop and web apps, any GTK-based file managers like Caja, Nautilus, Nemo and Thunar, you can drag and drop without problems.
I detest all these GTK-based file managers because they have a poor CLI command line, principally tabs. In the terminal, if I insert two paths to nautilus, nemo or thunar, they will open two windows instead of two tabs. Different of them, Dolphin opens two tabs instead of windows.