So GPU scaling isn’t available (as in, the option is greyed out)? Ok, that means it’s disabled. That means your monitor is probably deciding how to display the images. Go into your monitor setup using the buttons on the monitor and look at all the available options. There should be an option you can change that’ll make low-res or non-widescreen images stretch, just like on a high definition TV.
If that doesn’t work, you’re going to have to be more specific and describe what exactly you see in the Nvidia control panel; a screenshot would be very helpful. Like this: