Damn, I remember reading his blog post a few years back when I was an undergraduate procrastinating on lab reports. I was instantly in awe of his writing and latex skills.
From someone who apparently knew him personally^1:
> I just want to tell everyone that Gilles Castel, the incredible person that popularized this form of math note taking, has passed away. It's awesome to see that people still use his blog and keep his legacy alive. We lost him at a very young age. Thank you for this video.