How does 'phosphorylation of glucose' maintain concentration gradient in membrane transport (facilitated diffusion)?
Sort of a "magic trick" that biology does.

With facilitated transport, the movement is passive. That is, you have a protein that certain molecules/ions can pass through, but you aren't doing any pumping or using energy to move molecules. That means that the molecules are free to move in either direction, according to concentration (and electrical) gradients.

Therefore, assuming no influence of electrical gradients, the highest concentration a passively transported molecule can reach is equal concentration inside and out. 

Chemically, phosphorylated glucose is not glucose. It is phosphorylated glucose. If you phosphorylate every glucose molecule that comes in, your internal concentration of glucose would stay at zero. If phosphorylated glucose can't leave via the same facilitated diffusion pathway, then glucose will effectively only come in via that pathway, it can't go back out. The result is that, although glucose inside can never be higher than glucose outside, the sum total of phosphorylated glucose + glucose inside can continue to rise above the concentration of glucose outside.

Of course, this step isn't exactly free, because it costs the energy of phosphorylation.