Hopefully this can be implemented in the future. I have to say that I have gotten used to GUI applications on Linux to support portals for open/save dialog.
I'm using this on Fedora+KDE+Wayland for example with Evolution, Visual Studio Code and Joplin. All are installed "native" (RPM). No sandboxing (Flatpak, Snap, Toolbox container). One is a GTK application, the others are Electron-based. In all cases I just set the environment variable "GTK_USE_PORTAL=1" and the applications automatically use the KDE open/save dialog. Also most Flatpak applications that I have installed support this. Any program that shows the GTK/GNOME dialog these days feels a bit out of place on my desktop and can sometimes disrupt the workflow.
Anyway Geany looks like a really good editor. I will definitely check it out more. Always on the lookout for alternatives to VSCode. Thanks for all the hard work putting into it.