The Gradient Widget allows you to add your own CSS code for custom gradients.
To learn how to add a Widget to your Composition, click here.
As seen above, select the "Gradient" tab in the Property Panel to update this widget's following property types:
CSS - Box for pasting in the CSS of your desired gradient. Select from one of the three buttons on the base of this box to generate CSS of your desired gradient. You can copy the CSS and paste it back into this box.