Instructions
Go to the Smithy in Tir Chonaill and speak with the non-player character Ferghus. He will be able to upgrade the Gladius for the first 12 available upgrades.
Ask Ferghus to upgrade the Gladius, and he will require you to provide him with a specified amount of gold. As a general rule, the amount of gold required for an upgrade increases after each upgrade.
Have Ferghus upgrade your Gladius for the first 12 levels.
Speak with the non-player character Ranald. You will run into him in Tir Chonaill, standing outside of his martial arts school.
Pay Ranald the 34,500 gold he requires for the upgrade, and he will upgrade the Gladius.
Speak with the non-player character Nerys. She will be at the weapons shop in Dunbarton.
Give Nerys the 22,100 gold she requires for the upgrade, and she will upgrade the Gladius.
Speak with the non-player character Nicca. He will at the weapons shop in the Qilla Base Camp.
Pay Nicca 16,200 gold to upgrade the Gladius to increase its damage and critical chance.
Pay Nicca another 27,700 gold and he will upgrade the Gladius to its maximum level.