If your photogate turns on by becoming conductive, that is correct: when the switch is non-conductive, the pull-up will pull your pin to 5V, and when it is conductive, the pin will be pulled to gnd.
If i understood correctly and it is a Silicon Controlled Rectifier with the gate connected to a photodiode, that should be the case but you can expect some leakage current which means the pull-up will have a voltage drop across it... but normally not enough to make a difference for a digital read.
By the way, there's another guy on this board trying to make a camera flash controller who'd love to get his hands on your opto-isolated SCR!
