What Billy Kerr has so correctly outlined doesn't lend itself to sharing a HD image of file... He's saying you set up a simple gradient fill on one layer, create a new layer above, and then create a simple pattern fill on that layer with diagonal lines. In Photoshop, you would choose Edit>Fill, and in the subsequent pulldown for what you fill with, move from colour to Pattern. Photoshop ships with a boatload of patterns to begin with, and what Billy is saying is that this is just a basic diagonal line pattern which is in the Web Patterns library already with generic Photoshop.
Here's what that looks like in process:
Once you have that pattern fill on the layer above your purple gradient, you can choose one of many methods to get your overlay effect: as Billy suggested, you can invert and use normal transparency, or you can (as I did) leave the pattern fill as-is and choose the Multiply transparency mode, in my example set to 25%.
The nice thing about this approach is that it remains easy to reapply the gradient on the underlying layer with either a different direction (as shown in your envato examples) or a different set of colour stops, all whilst keeping the same graphic language by dint of your pattern fill over the top: perfect for multiple related pieces.