Three.js now operates in Linear sRGB color space and needs the texture color space to be set correctly. See the Updates to Color Management in three.js r152 thread.
You now need to explicitly set the color space of the canvas texture. This is unfortunately not something three.js can easily do for you in all cases since the correct color space depends on how it will be used and the contents of the texture:
gradientTexture.colorSpace = THREE.SRGBColorSpace;