The Secret Club's Secret is a side quest in The Legend of Zelda: Breath of the Wild. It is a quest given to Link by Greta in the Gerudo Town. It is one of the side quests related and needed to be completed for the main quest The Thunder Helm.
The Gerudo Secret Club is only open to those who know the password. Link must find someone who knows the password to gain entry.
To get the password, Link must travel across Gerudo Town to The Noble Canteen where he will find Pyra, Sumati, and Yaido chatting on the couches. If spoken to directly, they will yell at Link for eavesdropping on their conversation.
If Link visits the house next door where Gerudo child Pearle lives with her older sister at night and speaks to Pearle, she reveals that one can overhear people talking in the bar next door. If Link stands at the nearby window in Pearle's home, he can listen into Pyra, Sumati, and Yaido to their conversation secretly without getting caught. After listening to their conversation, the password will be revealed as GSC◆. By returning to the Gerudo Secret Club and giving the password, Link gains entry to the club. If Link inputs the password incorrectly, however, he will have to wait at least one day (in-game time) to try to input the password again.
Imputing the correct password causes the door to open and Greta allows Link to purchase "voe" armor at her shop. It is revealed that the password is due to the fact the Gerudo Secret Club is a black market shop that sells "voe" clothing as it is illegal for Gerudo merchants in Gerudo Town to sell voe clothing (this apparently does not apply to travelers as Link can sell any sellable armor to merchants in Gerudo Town with no problem), despite the demand for it. After completing this side quest, the door to the Gerudo Secret Club remains unlocked allowing Link to enter at any time. As it is a black market voe armor shop and Greta is not fooled by Link's "vai" disguise, the Gerudo Secret Club is one of two places in Gerudo Town where Link can remove his "vai" disguise and wear other clothing allowing him to try on the armor he purchases here, however he will be arrested if he leaves the Gerudo Secret Club if he is not wearing his disguise. It is the only shop where Link can purchase the "Radiant" armor set, as the "Desert Voe" armor set can also be purchased from Rhondson Armor Boutique in Tarrey Town after recruiting Rhondson during the "From the Ground Up" side quest.