It really depends on the texture of the cream you are after. If you want a creamier, more curd-like texture - or want to make the tart vegetarian - you can absolutely skip the gelatine. I personally like a filling that is slightly more set when I slice the tart, so I add a little bit of gelatine to it - about 1 teaspoon. It isn't much, so you will still keep a very creamy texture, but it is enough to have a more set texture. As always, it is completely up to you!