The circuit is a little confusing because there are two inputs to the noise gate. The plain guitar signal, before the distortion stage, is fed to the gate chip's level detector. This is the path to pin 1 via R30, C20.
The distorted signal is then fed to the actual gate part of the chip via C1, R5 into pin 17.
So, my first line of attack would be to unhook C1 from IC2b's output, and hook it up to IC3a's output instead, so that both the level detector and the gate get the same signal. Then you can get rid of IC2 and all the parts that go with it.
This circuit also seems to include some EQ. TR1 and TR4 are gyrators of the kind used in graphic EQs.