If you take a look @ descr_mercenaries.txt in \Medieval II Total War\mods\Third_Age\data\world\maps\campaign\imperial_campaign you will see all the mercenaries in-game, for instance ents for elves, dala cavalry for dwarves, beornings in Anduin province.
Gondor hasn't any mercenaries, only cogs in coastal regions.
See it as a sort of papal states in vanilla M2TW. As an evil faction try to please them (or Sauron in this case) which is more beneficial, unless you want to be an invasion target. The faction is located at right top corner of your map, but if you play by the rules; you won't spot him.
