It is the same thing. Gain and volume, those are identical processes in the software domain, both are multiplications of the samples with added dissonance. The only difference is that putting the gain as the first pre-fader plugin will allow the incoming signal to be adjusted in level before the signal hits effects routed on the channel and depending on your gain plugin moves that can then lower the output quality of those effects. A gain increase on such a gain plugin followed by a volume decrease with the volume fader, or the other way around, is a pure waste of signal. Therefore, by default it is not optimal having to adjust the level before the mixing process starts. The volume level on each sound source should be as close to the final level as possible in terms of balance and gain, with enough headroom on the channels consuming the most signal. Pulling the volume faders down is the worst thing you can do.
Strictly speaking of gain staging (incl. panning), it is most optimal to route the signal to the hardware domain.