With Photopea, you can let the application load, then disconnect from the internet (or otherwise firewall the app) before loading your image.

I just gave it a try and I could load the image, edit it in various ways, and save it just fine. If there are any features that require being online, I didn't find them.

What's stopping GIMP from doing the same thing? It has more access to the network than your browser.

Same with me, slight paranoia, but there's nothing to stop a native app from doing it either.