Hi, did you set the z-order of your sprites? For a seamless road the z-order for the left sprites has to be higher. You can set that in the properties window for each sprite.
Btw, making your map (especially an isometric one) with tiles in Gdevelop is quite tedious and not without frustration. Making the basic map in Tiled is probably a more pleasant experience.
Edit: I just realized that you asked about the grid and not the sprites, so what I wrote is not related to that.
Edit 2: If this is what you actually want:
you have to set the grid to a height of 50 for your sprites.