If you are registered at Sourceforge you could potentially post a new ticket here.

AFAIK the Windows dev is no longer active. But the main dev might be able to help.

Edit: Have you installed any security software or updates in recent times? There is a small chance a security app (possibly 3rd party) is preventing GTK opening. This appeared to be a relatively common occurrence several years ago with p2p apps being considered non trusty by such security apps (their devs were anti-p2p file-sharing.)

Also keep in mind afaik GTK-G might not be a properly signed app, so you might have inadvertently made a selection when attempting to open that has become default behaviour to disallow.

You might also try running the app from a different log-in account.

You should exhaust all possibilities before posting a ticket at Sourceforge.