Simple OpAmp Question
Assuming Vs is a conventional 0\$\Omega\$ source impedance independent voltage source, the gain is -\$\infty\$. 

All the resistors can be ignored since they are connected between 'stiff' (0\$\Omega\$ impedance) voltage sources. 

Imagine the input Vs is a very tiny voltage (say +1uV or whatever you consider to be inconsequential). The differential voltage the op-amp sees is -1uV. It will multiply that differential input voltage by an enormous gain giving a huge negative voltage (perhaps limited to the supply rails depending on how 'ideal' the imaginary op-amp is). Same thing if Vs = -1uV except it will result in a huge positive output voltage. 

There might be some kind of argument that it's just a huge number and not infinity since the resistors could pass enough current to overcome the zero source impedances, but I'll leave that kind of thinking to philosophers and pure mathematicians.