Elliot's talk on FRP can be found here.
In addition, not really an answer but a remark and a few thoughts: somehow the term "functional GUI" seems a little bit like an oxymoron (pureness and IO in the same term).
But my vague understanding is that functional GUI programming is about declaratively defining a time dependent function that takes the (real)time dependent user input and produces time dependent GUI output.
In other words, this function is defined like a differential equation declaratively, instead of by an algorithm imperatively using mutable state.
So in conventional FP one uses time independent functions, while in FRP one uses time dependent functions as building blocks for describing a program.
Let us think about simulating a ball on a spring with which the user can interact. The ball's position is the graphical output (on the screen), user pushing the ball is a keypress (input).
Describing this simulation program in FRP (according to my understanding) is done by a single differential equation (declaratively): acceleration * mass = - stretch of spring * spring constant + Force exerted by the user.
Here is a video on ELM that illustrates this viewpoint.
