What is the amplification when both input terminals of an opamp are at same potential?
Gain is a property of a circuit, not a signal. We normally consider the gain of an opamp to be independent of its inputs. For an ideal opamp, the gain is always infinite.

You're probably confused because your math is wrong. If you do this:

$$V_O = A({V_+ - V_-})$$

Then you can divide \$V_O\$ by the differential voltage to get the gain. But it doesn't work when the output voltage is zero:

$$V_O = A(V - V) = A(0) = 0$$
$$A = \frac 0 0 = \mathrm{undefined}$$

We expect something like this because when the input and output are both zero, the gain could be anything! Infinity, 100, 2, -5...