Unless prevented from doing so, gradients tend to equalise. Objects roll downhill, tea gets cooler, and tyres deflate. The same is true of concentration gradients. If you remove the membrane separating the glucose and water compartments, the glucose will diffuse into the water until the glucose concentration in the tank is equal throughout. The glucose is said to have diffused "down the concentration gradient". The greater the difference between the concentration of substances, the steeper the gradient and the quicker the the rate of diffusion.