Out of the box, there are no variables for the "pressed" gradient, and even if you created the variable yourself, the default "pressed" class is setup in SASS to handle a single color. That doesn't mean, of course, that you can't roll your own.
Here's what I'd do. Start by looking in resources/themes/stylesheets/sencha-touch/default/widgets/_list.scss. This is where the core classes for lists are defined. Make a note of the section where the .x-list-pressed class is setup, as well as .x-list-selected (this is the one you'll probably want to emulate). Don't modify this file--you can override it in your own custom scss.
Now, in your custom .scss, you can simply define .x-list-pressed as you'd like it. In this example, I just used the existing $list-active-gradient and $list-active-color, but you could always create your own custom color and gradient for .x-list-pressed and pass them.
In the following example, I've set up .x-item-pressed to look exactly like .x-item-selected. Swapping out $list-active-color and $list-active-gradient with custom colors and gradients for the "pressed" state should be trivial from this point.
@include background-gradient($list-active-color, $list-active-gradient);