How to create a pattern gradient in Illustrator

this doesn't really look like a traditional opacity mask job to me as there isn't any actual fade, its more to do with the gradual change in size and spacing of the shapes in the pattern. an opacity mask would be a great way to deal with fading from 100% to 0% in tone of a pattern, but could probably be achieved with easier future adjustments by just 'filling' a pattern with a gradient colour swatch?

there have been a few threads recently that looked at halftone patterns and one guy showed a way to use different shapes (other than default dots) as part of the halftone pattern which would achieve the result of the top half of this design. you'll have to search a few halftone pattern threads to spot it.

as a simpler manual option you could just draw the shape at the top, then scale and adjust one at the bottom and use a shape blend to make one 'column' of the pattern, then repeat it across the page.

the dotty part at the bottom looks more like a traditional halftone pattern, Phantasm plugin allows great control over things like this, or you can just play around with a black and white radial gradient as a fill to a circle, and use the half tone pattern effect to create the dots.