Once that's set up, you can save the FX Chain as default, meaning every new track you create will have a Gain trim knob on it. And as long as you put all your inserts after it in the chain, it will work as you expect.
Also, if you enable "Show FX Parameters" in the Mixer, the new Gain Trim knob will be visible without having to open the FX Window
I just use the input control on the first guitar effect plugin in the chain, usually TSE 808/TS 999, if I'm not using one of those it's the input knob on the amp sim.