Guard Ring on a printed circuit board for a transimpedance amplifier
The guard needs to be a low-impedance potential in order to protect what it is surrounding. Using a capacitive-divider like you have is ... not great. To generate a low-impedance mid-scale reference, use a resistor-divider between +VCC and -VCC, and then bypass that with a capacitor to -VCC to filter out noise, and buffer that with a low-noise op amp:



simulate this circuit &ndash; Schematic created using CircuitLab
You can then use this low-impedance Io to drive your guard ring.
A couple more notes about your circuit:

You will definitely need a capacitor in parallel with R1 to prevent the AD795 from oscillating. A 1pF cap is a good place to start, but it really depends on a combination of: the feedback resistance (R1 == 100Mohm); the opamp pin input capacitance (2pF); and the capacitance of whatever is connected to INPUT (TIP)
If you end up placing the input protection diodes D1 and D2, make sure that they are both low-leakage and low-capacitance. Even though your guard trace surrounds the input node, you'll want to reduce any capacitance between it and the input. Otherwise, you'll hurt your amplifier's frequency response
R2 will cause a slight voltage difference between your guard ring and your input (TIP), which could cause current to leak directly from your input to your guard trace. I don't know what source will be driving your input, but if it's anything like a photodiode, you don't need the resistor there and would be better off removing it.