On the Ventures where you send them out for specific items, yes, more Gathering can improve the amount gathered, and I believe that factors into the time spent as well.

For the 18 hour non-specific Ventures, and the Quick Ventures, no, Gathering does not improve the number of items retrieved.

Perception has no bearing upon your Retainers.

For the combat Retainers (DoW and DoM classes) its based on ilevel, and not Gathering.