Grunge effect can be done pretty easily in Galaincha by following the steps below.
Step 1 : Add a new layer over the design.
Step 2 : Select two similar colors as used in the design as primary and secondary color and go to CarpeTools>Create>Irregular>Contour.
In the Contour window, set the Scale, Roughness and Steps as per your need. And click on OK
Step 3 : Now select the Paint Bucket tool from the toolbox and select transparent from the color toolbox on the left side of the window.
Now holding down the Shift key, click on the design. and you will get the Grunge effect in your design as shown below.