Hey Tonyx,
Your circuit is connected similar to the following schematic.
The gain of this amplifier is going to be -RF/RG, where RF is your feedback resistor, and RG is your input resistance before your signal. R3 is your RF, but there is no RG resistor placed there. This means your RG is just going to be whatever parasitic resistance is between your input signal and your op amp. This is a very small value. if you divide 1GOhm by a very small number, you get a very large gain. This means your op amp is going to try to amplify your signal by -1GOhm/0.001mOhm, which is essentially open loop.
If your signal on IN- is above your reference voltage, your op amp will output as close to 0 as it can. If your signal is below your reference voltage, it will output as close to the supply as it can.
Best,
Jerry
