It might be an interesting question what this means for GADTs which _are_ functors in the Haskell sense, but I suspect these GADTs are already trivially reducible to plain Haskell data types.

My notation was somewhat ill-defined. K_S was intended to mean the constant functor over the discrete category generated from S, S is not being passed to it. It is just K1 : S -> Set. I didn’t want S and T to completely disappear. (Also, in Lan_H o Lan_K, I meant arbitrary H and K, I just wasn’t thinking there.)

The representation of the (semantics of) containers as left Kan extensions comes from “Constructing Polymorphic Programs with Quotient Types.” However, you can also arrive at it by using the explicit representation in terms of coends. Using the names from the explicit representation for Lan_J F, F = K1 and the equivalence ~ is just the identity relation since we are “integrating” over a discrete category. The coend becomes just the dependent sum and the product trivializes to just the left component.