Parallel RC filter on SCR gate
R2 is a "gate-cathode resistor".  Visually, the schematic makes it look like R2 is about C2, but it's really about the SCR's performance.  A gate-cathode resistor allows some of the SCR anode current to bypass the cathode, making for stabler operation and better high-speed performance.

See, for example, page 1-9 of this app note. For more informal description, see this web page.


  The action of the resistor can be seen with respect to the two transistor analogy of the SCR. It shows that a low external resistance between the gate and cathode bypasses some current around the gate junction. Accordingly a higher anode current is required to initiate and maintain conduction. It is particularly found that low current high sensitivity SCRs are triggered at very low current levels and therefore an external gate-cathode resistance is required to prevent triggering by thermally generated leakage current in the gate region. However the gate cathode resistance bypasses some of the internal anode current caused by the rapid rate of change of the anode voltage (dv/dt). It also raises the forward break-over voltage by reducing the efficiency of the NPN transistor region thus requiring a somewhat higher avalanche multiplication effect to initiate the triggering. The current that bypasses the gate junction also affects the latching and holding currents.
  
  It can therefore be seen that the effects of using the gate cathode bypass resistor include:
  
  ◦   Increase the dv/dt capability.
  
  ◦   Retain gate damping to assure the maximum repetitive peak off-state voltage VDRM capability.
  
  ◦   Lower the turn-off time, tq.
  
  ◦   Raise latching and holding current levels