Add Octane Gradient to selected node

[SSmolak]

Octane Gradient is the most commonly used node. Please add Octane Gradient to "add to selected" menu just like Composite Texture so it will be automatically added to selected node output.

[elsksa]

And default to grayscale/floating node instead of RGB, since it's used in a greyscale manner nearly exclusively.

[elsksa]

And default to grayscale/floating node instead of RGB, since it's used in a greyscale manner nearly exclusively.