Which units to bring
The only units that give you a boost to gathering are Serfs and they max out gathering bonuses at level 7.
You can have 2 setups for gathering, either bring 5x lv7 Serfs for maximum gathering capacity.
Or bring 4x lv7 Serfs and 1x Strong unit to defend if someone attacks you in open world, if they win you lose half your loot.
At each site you have to pay Taxes or Tariffs as they are called.
These taxes accumulate and get converted into silver for the house that owns that particular gathering site.
The house can set their own tariffs so be careful, some sites that should only cost around 2000 to gather at might cost 20 000 because the house owning it not wanting you to gather there.
A way to go around this are the Requisition tokens that we get from daily logins as well as daily quests.
These let you completely ignore the tariffs.
which sites to gather at
You should gather at sites that has the required resources you need to complete high level fief quests.
You should also gather at sites that has the required exotics for whatever unit kits you want to make.
To learn which unit kits need which resources head over to Troop Guide under the Troops tab.
Below you'll find a spreadsheet over where to find each resource.