Hello!
To quote the referenced post:
After creating the this node you would go to its node properties and set the Mapping option to Spherical. Your gradients would be saved into textures and then set as the Top, Middle and Bottom textures of the Triplanar node.
This means that the gradients would have to be saved as textures so that they could be used by the node, since the node only accepts Texture Object nodes in order to receive textures as the texture input.
If you're looking into setting up a gradient, you may want to refer to
this post, although it usually is ideal to simply use a color ramp texture.