Sensitivity measures the voltage at the output pins of the mic for a given sound pressure level. 13 mV/Pa means that an air pressure oscillation with an amplitude of 1 pascal (94 dB SPL) at the mic capsule would produce a voltage oscillation with an amplitude of 13 mV at the XLR pins.
Gain is basically the ratio of output level to input level.
In this case we're not talking about an input and output, but you can still talk about the relative gain difference, which would be (25 mV)/(13 mV). The sound pressure level (1 pascal) is the same for both, so it cancels out. The relative gain difference is then 25/13 = 1.9, or about 2x as much voltage for the same pressure.
In decibels, the difference would be 20·log10(V1/V2), so +5.6 dB for your example. A doubling in voltage is about 6 dB, so you could guess this just by recognizing that 25 is almost twice 13.