Hi Fj, try to make the gradient whit another soft, in my case the gradients make in photoshop don´t works well, but makes in 3ds max in .png 16bits works well for me.
The photoshop make in some cases things wrong, another example is the efects, if you make a glow whit PS, don´t have transparency on Iphone, but make on max or another soft 3d, the glow works well, the output format it the same, but I thin kevery aplicatión have his own behaviors about the outputs formats.
Best Regards,
Iván J.