I want to connect two gradients at a 45° angle:
As you can see, the transition is not smooth. How can make the gradients blend properly?
Obviously you have filled two areas with a black-white-black gradient. In theory you can nail it, if the seam of the filled areas is just in the right angle. The ends must fit like they fit in high quality woodwork.
Fill a rectangle with the gradient in a new layer. Duplicate the layer. Rotate the duplicate with the layer rotation tool:
Move the duplicate, make lower corners to fit:
Click a lasso selection and delete the extra part in the duplicate:
Beware: If your gradient isn't exactly symmetric, you must rotate to the right direction to make the gradient fit, counterclockwise in this case.