I've used GOG Galaxy app to download witcher 3 GOTY edition, it shows disk read error failed when download had completed. I'm unable to fix to issue by clicking verify/install.

How do I go about correcting this? I am not good with computers. I use windows 10

If it really is a disk read error, and not some bizarre Galaxy error, then that's bad news, because it means your hard drive is failing. You should beck up everything important on that drive NOW, then get it looked at by a professional.