I don't know the Geany code infrastructure and if such a functionality would be implementable through a plugin.
Unfortunately there is currently no method for a plugin to intervene in the file open process. Open is a fairly complex process and there are many places where a plugin could be called for various things, and you would have to get just the right one for this use-case.
The best that could be done at the moment is for the GPG plugin to have a separate open menu/button/shortcut which saves an unencrypted temporary file and calls Geany to open that and after Geany closes and writes that re-encrypts and deletes the temporary file.