(SD Notary was notarized by itself.) It will notarize. Although it was designed with script applets in mind, it should work with any application that has relatively straight-forward requirements. SD Notary is an app that wraps a more friendly user interface around these tools. Some of these tools, though, are still actually part of Xcode.
For apps not written in Xcode, such as script applets, notarizing can be done using command line tools. Most macOS applications are written in Xcode, and the process is designed with that in mind. The basic process involves three steps: a more comprehensive form of code-signing, submission to Apple where the app is checked and approved, and (optionally) stapling the resulting ticket to the app.
With macOS 10.13.6, Gatekeeper is requiring notarized apps where it previously accepted simple code-signed apps. SD Notary is a utility for having apps notarized by Apple.
