Most of Gondor's units were designed for defense (Tower Guards, Soldiers). Their primary offensive units are the knights. Rangers are meant for hit-and-run tactics and archers are best standing on walls with fire arrows.
Gondor's hero choices are more limited as the choices are Boromir, Faramir, Gandalf, and Pippin. Pippin is the cheapest, then Faramir, Boromir is after, and Gandalf is the most expensive.
Pippin is best for scouting and building outlying farms, and neutralizing light resistance, such as orc warriors.
Faramir is a versatile hero who can use a bow or sword. At Rank 3 he can mount a horse as a knight. His leadership ability give his troops an armor bonus.
Boromir is a heavy soldier, well equipped to deal with infantry or creatures. His leadership gives his troops an attack bonus.
Gandalf is a heavy hitter, with abilities that can destroy infantry, creatures, structues, or, at later stages in the game, entire armies.