you can tell the material to mask wherever you like in the uv space. then you just need to make sure your meshes uv are setup in a manner that works (or reverse the order and make material work for mesh uv).
below is a few examples. first if you wanted a linear gradient theres a node for that. the node will give you a fade from a single direction say top or left, from there you need to dulicate and negate that fade using a 1-x. after that i included some functionality to modify the gradient size and brightness. once you have this setup simply plug it into the opacity.
another option is to use textures as the mask. the texture mask i created wasn;t in the right orientation so i used a rotator to rotate the uv. from there it can be as simple as plugging in a texture to the opacity. i decided to add some variation by including some macro/micro tiling.
