Current Gate in AGC is almost useless because it's before AGC. So everything about gate in AGC directly depends on input level. Also Noise Gate..
Isn't that the point?. AGC's job is to put what's presented at the input to a level that everything following can work with. If the gate was set at, say, 30dB below its own output level, then got stopped after some loud passage, it could fail to open back up on something following that was at a reasonable, but low level. The opposite would be true, possibly making the gate more sensitive than intended. Your thought works if something preceding ST keeps levels within a reasonable window... like an AGC before the AGC... but I have a hunch the action would be inconsistent.
Maybe Hans could put yet another option in there (after/before), but I have a hunch it would probably work better as it is.
I assume the following compression filters do reference their action, based on input, but that window has already been narrowed by the AGC. In the case of the second multiband, it would be shortened even more by the actions of the first.