Gats uses Stumpy Pencil v2, a Photoshop brush preset.
http://stumpypencil.blogspot.com/2009/11/return-of-stumpy-pencil.html
The handy thing with brush presets is that all your tools will have the same texture and dynamics so there's a consistency in style even when erasing.
To apply a brush preset, go to the Photoshop toolbar.
Window > Tool Presets
under that menu's dropbox, select 'Load Tool Presets…'
From there, navigate to the Stumpy Pencil v2 file you've downloaded from the link, and there you have it.
You could always use the binary tool in SAI as an alternative. It isn't Gats's style, but still looks alright.