Welcome to our forum and thanks for reporting. Usually, when the installer is finished, Gravit Designer should pop up and an icon should be created on the desktop. You should also get a dialog, that asks you whether Gravit Designer is allowed to make modifications to your system. Maybe you missed that?
Does the portable version work for you, when you unpack everything into a folder and run it there?
For me, Gravit Designer installed in “%localappdata%\GravitDesigner\app-3.2.6”.