The GB Instruments are part of GB so they can only be run within GB. Some third-party instruments are available in both plug-in formats—which will work only within a DAW (like GB)—and standalone format—which will open on its own as an application. If an instrument is available as a standalone then yes, you don't need to open GB to use it—after installing it the standalone version will be found in your Applications folder and will have its own Preference window where you can make MIDI & audio interface settings. But if it's only available in plug-in format, then you would need to be in a DAW (GB) to use it. In the case of Helm, apparently it comes in both plug-in and standalone formats, so you should be able to open & run it as an application, outside of GB; other instrument plug-ins may or may not offer that same capability..