Glad to oblige, for K:

    S..t:".[`D;(;);{. y};S[]];S[.;`f]:9$D[]"
    S:D:.+(`$'_ci 97+!26;26 99#,"")
    `show$`S

This K (K2/K3) version of a simple spreadsheet (so demoing GUI, "reactiveness") is also mentioned in the original post's comments.

So, I did this: https://gist.github.com/maximvl/8d4cd6c9139f6f8f3729f4ba59fa... and nothing happened. How do I run this?

Seems you're running Kdb+ (also known as "Q" or "K4"). You need K in the older K2 or K3 version to run the above. Kx stopped selling K2/K3 so they are basically no longer available.

I see, well, it's good that Red is open-source.

A spreadsheet in k3, demonstrating gui and reactive capabilities: http://nsl.com/papers/spreadsheet.htm