Can you try forwarding the emails from gmail to Outlook and check if they are fixed after it?
If this solves the problem, you could set up an auto forwarding filter in gmail as a workaround. In gmail the attachments should never appear wrong.
Unfortunately, Outlook may have complex configurations or bugs and we cannot know the exact reason of corrupting the attachments. It could depend on what kind of attachment are you sending (image, PDF, etc).