adding gradients to swatches panel

You cannot add gradients directly to a CC library, but as a workaround you can drag an object using a brush/pattern/gradient/global color to the CC Library. 

You can also drag multiple items using different brushes/patterns/gradients/global/spot colors together into a CC library to simulate a library.

The easiest way to add them to a new document is to hold down the Alt key when you drag them from the library and place them in a new document. The patterns/gradients/global colors automatically get added to your swatches panel.

You can hit escape before you click to place the object(s) or delete the dragged library items because the swatches and brushes are now part of the document.

The swatches you see when you create a new document are part of the New Document Profile that is used as the preset for that document. If you want to have your gradients in there, you need to change the New Document Profiles.

https://helpx.adobe.com/illustrator/kb/create-custom-new-document-profile.html

You can also save a Swatch Library with your gradients and make it persistent. To make a swatch library appear each time Illustrator is started, select Persistent from the swatch library’s panel menu.

To create swatches that are available in the swatches panel in every new document you create, you need to edit document profiles. For some background info and instructions, please read:

https://happyrainbow.art/2019/06/06/document-profiles-in-illustrator/