Solid State Relay using photovoltaic MOSFET driver
I could only imagine this circuit working if a resistor is added between T1 collector and base in order to bias the base of T1 (let's call it Ry), and PC being a phototransistor. 
Given this assumption, then R6, D4, and C1 act as a &quot;voltage&quot; buffer.
When no signal is given to A1, PC is open, T1 conducts biased by the added resistor Ry. Z1 protects the gate against over-voltage and the current is limited by R6. D3 and D5 avoid current flow back to PC. T2 base voltage is high so it does not conduct. D5 flows current to T2 base and the R5 so it does not conduct. So the MOS gate would be high.
When a signal is given to A1, T1 base is low as shorted to the minus, current is limited by R6 and Ry so T1 does not conduct. T2 base is low as current can flow through R5, with D5 avoiding flowback to PC, so it brings the MOS gate to low.

Of course, a simulation would help.
EDIT:
As ElectronS pointed out, when active, the MOSFET voltage drops to 0 thus will remain in that state.
The whole point is PC, which seems to be some sort of Photovoltaic device comprising many diodes in series, generating a voltage / current high enough to trigger T1.
This device could be a candidate with a voltage output up to 8V and a short circuit current of a few uA, however you need to consider the actual current at the required voltage. T1 would need to have a fairly high gain.