Why this gasps in floor is appeared?

This is very unpleasant thing for me.

What could it be?

if those are separate objects, then the precision of floating point co-ordinates is never perfect. you have a few options… 1) weld those objects together, (MeshBaker tool) so they share the same (imperfect) vertex co ordinates… 2) make the scale of the objects a very tiny bit bigger than 1.0 so they overlap just a little. 3) make the background color less contrasty.