Hi, I'm not the original creator of this style but I used the text characters they used to make the most recent Chrysalis ASCII post.
I created my piece using google docs Document as my canvas, starting with a copy of this template:
ASCII Block Art Template
The process is similar to creating pixel art... one pixel at a time. It helps to have a reference on the side.
There might be a way to create a brush in Gimp, and I know there is a way to create a brush of these characters in Photoshop, however you obviously won't be able to post the image version of those as text in comments.
Finally, something to note: based on the user's browser settings, webpage, and font settings you used to create the ASCII art, underscores "_" may vary in size, and distort their line's offset in the final "image."
Good luck!