simple linear gradient
Linear gradient function is taking texture coordinate, masking out U(R) and V(G)
U gradient is from left to right
V gradient is from right to left
Here I'm using a Switch parameter node to switch between u gradient or v gradient.
Then multiply to a scalar parameter for direction. When using U gradient, this would allow us to switch between left and right. When using a V gradient, this would allow us to switch between up and down.
Then Subtracting the gradient to a scalar parameter would allow us to do the dissolve effect.
To make the transition more interesting, subtract a noise texture.
Quick Demo
2. Sphere gradient Control