Protecting circuit input against lightning - how to put all the pieces together?
I reckon the Trisil should be rated for a voltage exceeding the GDT
sparkover voltage, say 120V (or is that too low?)

EDIT

You can't use a Trisil with a GDT because as soon as the Trisil fires, it clamps down to a low voltage and that will never allow the GDT to activate. Now, this may work fine on its own (and you might not need a GDT) but it'll never handle the surge power that a GDT can handle.
The GDT slow-strike voltage is 75 volts plus 20% = 90 volts. So if you use a regular TVS diode, it will allow the GDT to eventually take the brunt of the surge after initially clamping the surge to circa 100+ volts.

Next, you don't need C11 if that's all you have. If you have an output amplifier you need to reveal that because the input amplifier will survive without any further TVS devices.