Some of the photos I take, to get the subject large enough in the frame I have to use electronic zoom. I don’t have money for a nice zoom lens. I tried using an adapter for one of Dad’s zoom lenses but it sometimes gives me issues. So I use a 4x zoom - which basically cuts off 3/4 of my sensor then expands the picture to full size (I guess by some sort of averaging math to create the discarded pixels)
Is there anything I can do in post to get some of that resolution back - even if it made up. I am a Linux user, so a workflow in GIMP would be great, or any other Free/Libre software you might suggest?
You will never get those pixels back. They are gone. You can, however, create new pixels. They won’t be as good as the originals, but in some cases, they’ll be good enough. An open source AI/ML interpolation tool I have used with decent results is called Upscayl. There are AppImages, Flatpaks, and many Linux packages available to download.