Google searching “gumroad hdri” brings up a lot of results. There’s plenty of tutorials on making them, either you need a nice camera, some extra gear and a tripod, or you can create a digital HDRI scene in 3d applications. How you go about making them varies a lot depending on the application.
It’s not malware. Works fine. Use some disposable e-mail address if you are really so very afraid. But I tried it for you and you can get a free skyboxes without any issues or spam.