For the non-vegan gluten-full baker

Yes. To substitute gelatin for the agar flakes called for in this recipe, use 1.5 tablespoons of gelatin powder.

I came at that number only after reading a comment in which the author said that 1 tablespoon of agar flakes = 1 teaspoon of agar powder.

I learned from reading this Food52 post about using agar https://food52.com/blog/17465-agar-agar-is-inconsistent-wily-mysterious-but-here-s-what-we-know
that you should use 2 - 3 times as much gelatin as agar agar. You want this tart to set up firmly, as you will be slicing it, so I suggest going with the 3X ratio.

If you try this, do let us know, please, how it turns out! ;o)