Hmm.. in theory (and IME) it should be the opposite actually.
Amps, drives and pretty much all non-linear effects in a typical signal chain all compress and distort the signal, so the delta between the loudest guitar level and the noise level gets reduced compared to the DI signal.
This means that the dynamic range is reduced so a gate placed after amps/fx would need a higher threshold and ratio settings to cut out the noise, with the risk of cutting out a good chunk of the guitar signal along with it.
Thus, IMHO, it's always better to use a gate at the beginning of the chain if its purpose is to reduce noise (in a digital device at least, in the analog world there can be other noise sources apart from the guitar and placing elsewhere could make sense).
PS: If you don't get what I'm saying just look at this extreme example:
Imagine you have an amp with a huge amount of gain and you keep turning it up.
At a certain point the guitar signal will be hard-clipped so its volume won't increase anymore, it will just get more and more distorted.
The noise noise in the meantime gets amplified till it reaches the same level of the guitar. At that point it would be impossible for any gate to separate the signal from the noise.