Goldmask's set is a unique armor set composed of four armor pieces: 
Radiant Gold Mask, 
Goldmask's Rags,
Gold Bracelets, and
Gold Waistwrap.
They can be obtained by following the questline of Brother Corhyn, a merchant and scholar who is obsessed with the Golden Order, a faction that worships the Erdtree.
Here Are the Steps to Get Goldmask's Armor Set:
First, you need to meet Brother Corhyn in the Roundtable Hold, a safe area that you can access early in the game. He will sell you some Incantations and tell you about his interest in the Goldmask.
Next, you need to reach the Altus Plateau region, which is located north of Liurnia. You can use the Grand Lift of Dectus to get there. Once you are in Altus Plateau, go back to Brother Corhyn and talk to him again. He will tell you that he is going to search for the Goldmask in the world.
After that, you need to find Goldmask on the north end of the Forest-Spanning Greatbridge, which is a Site of Grace in Altus Plateau. You can use a teleporter to get there. You will see Goldmask standing near the edge of the bridge, pointing at the Erdtree. He will not say anything, but you can talk to him and get a Radiant Gold Mask, which is one piece of the armor set.
Then, you need to go back to Brother Corhyn and tell him about Goldmask's location. He will be excited and ask you to join him in his quest to restore the Golden Order. He will also give you a Mending Rune of Perfect Order, which is a special item that can heal you and your allies.
Next, you need to follow Brother Corhyn and Goldmask to Leyndell, Royal Capital, another region you can access later in the game. You will find them in the Colosseum, where they will ask you to solve a riddle for them. The riddle is: "What is golden but not gold? What is old but not cold? What is hidden but not lost? What is given but not sold?" The answer is: "The Erdtree". If you answer correctly, they will reward you with a Golden Order Banner, which is an item that can summon friendly knights to aid you in battle.
After that, you need to follow Brother Corhyn and Goldmask to the Mountaintops of the Giants, which is another region that you can access later in the game. You will find them near a giant statue of Radagon, the founder of the Golden Order. They will tell you that they are going to perform a ritual to awaken Radagon and restore his glory.
Next, you need to progress the story until you unlock Leyndell, Capital of Ash, which is an endgame area that is covered in ashes and ruins. You will need to defeat some bosses and complete some quests to get there.
Then, you need to find Goldmask near the Leyndell Colosseum, where he was before. You will see his corpse lying on the ground, surrounded by light. You can loot his body and get his armor set: Goldmask's Rags, Gold Bracelets, and Gold Waistwrap.
Optionally, you can also find Brother Corhyn in Leyndell, the Capital of Ash. He will be near a giant tree that resembles the Erdtree. He will tell you that he has failed his quest and that he regrets following Goldmask. He will also give you a choice: either kill him or spare him. If you kill him, he will drop his weapon: Corhyn's Golden Sword. If you spare him, he will thank you and disappear.
By following these steps, you will complete the questline of Brother Corhyn and Goldmask and get items in Elden Ring. You will also unlock a secret ending for Elden Ring: The Age of Order. This ending will show you a cutscene where Radagon awakens from his slumber and restores the Golden Order with his power. He will also crown you as his successor and grant you his blessing.
I hope this guide helps you get the Goldmask's armor set easily. Have fun playing Elden Ring!