The problem you're encountering is that the Tile Sampler tiles the whole input node, not just the white parts of it. If you want to close those gaps, you'll have to scale your shape within the Tile Sampler so that they overlap.
Here you can see the result before and after. For the second Tile Sampler node, I have used the Size X Y parameter to independently scale my shape along the x and y axes to get the desired effect.

You can also do this with a much cheaper Tile Generator (always better to use when you don't need a specific feature of Tile Sampler). On its face, the Tile Generator node doesn't appear to have independent size controls for X and Y. It does however have Interstice controls. All you have to remember is that Interstice = 1 - Scale. So if you want your object to be x1.5 scale along the X axis, set the X interstice to -0.5
