Gain on a Differential Amplifier
The site gives you the output equation: \$ V_{out} = \dfrac{R_3}{R_1}(V_2-V_1) \$ Difference amplifiers find the difference between two signals, and amplifies the difference. The \$ \dfrac{R_3}{R_1}\$ factor determines the gain of the difference. The gain is relative to the difference only. If the two input signals are the same, the output will be very close to zero, depending on the common mode rejection capabilities of the op amp.