Using gradients in layer masks

I'm trying to add multiple gradients in one layer mask, which I was able to do before switching to beta. Now, it seems it will only allow me to apply one gradient to a mask. It's so frustrating! How can I add multiple gradients to one later mask? Example: I'd like to create a transparent gradient on multiple sides of an image. The first screenshot shows what it's allowing me to do. I can only apply one gradient to this mask. If I move to another side of the square to create another gradient, it removes the original gradient. The second screenshot is an example of what I'd like to achieve, using one layer mask (I had to create each gradient separately, apply the mask to the layer, create another mask, apply, etc to achieve this).