A fork of GNOME Schedule (ported to GTK3) is available at https://github.com/rizalmart/gtock.
Warning: The developer is working on implementing meson build system. Right now, the recommended method of installation is to copy the source packages to your /usr folder, which is dangerous if you type a wrong command. I have verified that this method works, but use this method at your own risk.
First, install cron, which is a dependency.
sudo apt install cron
Then, clone the repository.
git clone https://github.com/rizalmart/gtock.git --depth=1
Copy the relevant folder to your usr.
sudo cp -r $HOME/gtock/gtock-src/usr/* /usr/
Mark the relevant binaries and the .desktop as executable.
sudo chmod +x /usr/bin/gtock
sudo chmod +x /usr/share/applications/gtock.desktop
Finally, compile the glib-schemas.
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/
You can open it with the command gtock, or open Scheduled Task in the menu.