When drawing with the default gradient in a mask, if the mask isn’t quite right, it’s easy to draw another gradient to replace the first one.
If, however, you want to draw a secondary gradient that will add to or subtract from the mask (instead of replacing it), change the blend mode for the Gradient tool to Multiply (to add black) or Screen (to add white) and then drag the second gradient.
Note: in the example above, I selected Edit > Undo to undo the gradient that drew in the second illustration before changing the blend mode to Screen and redrawing the gradient.
Of course there are other ways to draw masks, but I find this to be straightforward. Plus if you use the radial gradient you can create a cool looking “bubble mask” by drawing multiple black to white radial gradients with the Gradient tool’s blend mode set to Darken – although I’ve never actually used a bubble mask like this for anything useful – but I’m sure that someone has!