Mordor has an immense unit roster, that needs to be catered to the opponent's unit composition. A couple of units that are very important against Gondor are the Cirith Ungol units. Cirith Ungol troops cost no CP, so make sure you properly spam orcs to fill the line and save enough resources to get some Halbeldiers, Black Uruks and Black Uruk Archers. Then, as you have levelled Sauron by probably recruiting a Nazgul and a Cirith Ungol unit, it is probably the smartest to go for Overseers. Overseers are absolutely crucial in lategame slogs, put them on the Cirith Ungol Halbeldiers and see them destroy Gondor Knights with their ability active. I would also try shifting around your build to experiment. Try arsenals and go Troll Cage or Siege Works. Both have purposes at dealing with Gondor. Trolls need to be upgraded, with either a sword or a mace, and catapults are good at poking at Tower Guards. Since Gondor is a defensive and slow faction, I would recommend going for Dol Guldur if you do not go for the Outpost. Poison is always useful against heavily armoured troops, and get Khamul with his armour if the enemy is camping in their fortress. Castellans are really good against Tower Guards too.
I see this trend where players seem to get only 1 or 2 infantry structures in their base, and then fill the rest with upgrade cost reduction resource structures. I don't get that. For example, instead of having a 5% extra discount on your upgrades, you can have a different recruitment structure that can complement your army greatly. The whole point is to have different buildings in your base so that you have numerous threats you can answer to. Especially in this case with Mordor, skip one Tribute Camp, since you have more than enough to upgrade for CPs, and get that troll cage or sige works that is supposed to complement your army. Spamming from a single type of structure gets old really fast if you understand what I am saying. You do need to try to get the timing right for these experiments, so I do suggest practicing with them.