What is concentration gradient?
This would be a region (in a solution) in which the concentration changes with distance.
A gradient in general is a rate of change of some quantity with distance (distance is the denominator in the rate). Examples include pressure gradients (which result in high winds), temperature gradients (which a well-insulated wall should maintain), altitude gradients (the slope of a hill).
So, a concentration gradient would refer to a solution in which the concentration changes over a certain distance. It does not necessarily suggest that the change must be uniform with distance, although it could be.
This might occur if you placed salt into a glass and slowly poured water over it. The bottom of the glass would have a high concentration of dissolved salt, while the top would high a much lower concentration.