Use a gradient, from white to transparent, make it on its own layer, as a unique object, and then export it as a .png, by itself.
You can do this vertically, or horizontally, then mirror/duplicate whatever you create, and put it on the other side, so you get that fadein/fadeout look.
Make sure you export as a transparent .png, unmatted, so that it's going from white to transparent, without anything else on... i.e. turn all other layers off before exporting.
Then, in whatever rendering/game engine you're using, add this over the top, preferably with an additive blend mode, angle it to your liking, and pass it across (animation) the object you want to have appear as though it's gleaming.