>With the transformer out however, the gate sees 65V (V+ = 310V).
>Using a 470R resistor as a light load instead of the transformer
>the gate voltage again drops to 200mV

This sounds like something in the trigger circuit is always turning on the 
SCR. You should be able to verify that this is the case by measuring the 
voltage between the 1n4007 and the opto. Also I would expect a resistor of 
about 10k between gate and cathode of the scr to prevent false triggering of 
the scr by stray emf/emi/induced hum.

I don't understand why he has the 1n4007 in there either. this seems 
unnecessary. 

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