Finding the gain with a photodiode
The opamp is in current to voltage configuration. All the current provided at the negative input node flows through R15 and becomes a voltage, the output voltage is then:
$$
V_{out} = R_{15}I^-
$$
Where \$I^-\$ is the current flowing into the negative input node. Please note that no current flows into the amplifier negative input, but some current flows into the associated node (and through R15 then).

Your gain is then:

$$
A = 5.1M\Omega
$$

I am assuming that your signal is at frequencies low enough to neglect C4, that guy is there to improve stability.