Glide storage is neither private nor secure. When you upload a file/image to Glide, it’s stored and hosted in Google Cloud and a URL is returned. Although the URL isn’t discoverable or easily guessed, it is a public URL.
A Private/Authenticated storage option is a long term feature request.
There are workarounds that you could implement. For example, you could transfer all files to your own Google Workspace or some other private storage location. But then you’re faced with the issue of allowing your App users to access the files. For example, if they’re using your App via public cellular or wifi network then they probably won’t be able to access the files, unless they’re logged into an account in your workspace, or connected via VPN, etc…