1. Edit panel settings

The only thing you need to do to achieve the desired result is to change the GIF Panel's settings so that the adorner frame is the only 'screen' in the settings.

Open the Settings script of the GIF Panel and locate the Screens setting.

Remove the existing screens from the setting so it's empty. It should look like this:

Screens = {

};

Now add the frame that's in the AdornScript inside of ServerScriptService. To get to it, you need to go into ServerScriptService -> AdornScript -> Frame, so the setting should be game.ServerScriptService.AdornScript.Frame

In the end, the setting should look like this:

Screens = {
    game.ServerScriptService.AdornScript.Frame
};

And that's all you have to change. Now your GIF should stretch seamlessly across the screens.