Sikhism has no clergy and no priesthood; any competent Sikh may read the scripture in public worship. Authority attaches instead to institutions and to the scripture itself.
The Guru Granth Sahib is not a book about the Guru but the Guru: Gobind Singh ended the line of human Gurus by conferring that status on the scripture, and it is treated accordingly: installed, attended, and put to rest each night. The Akal Takht at Amritsar is the seat of temporal authority, and its jathedar issues edicts that bind the community, though the extent of that authority is itself argued over.
The Shiromani Gurdwara Parbandhak Committee, formed in 1920 and given statutory footing by an act of 1925, is the elected body that administers the historic gurdwaras of Punjab, Himachal Pradesh and Chandigarh, including the Golden Temple. It controls very substantial funds and property, and it is a political institution as much as a religious one. Its writ is territorial, and there is no world Sikh authority, which is the root of most disputes in the diaspora, where gurdwara management is settled locally and frequently in court.
The standard code of conduct, the Sikh Rehat Maryada, was drafted from 1927, approved in stages through the 1930s, and finalised in 1945. It has had only minor clarification since.
Who counts as a Sikh for the purpose of electing that committee is a live question, and it is decided by legislation rather than by the community: an amendment in 2016 excluded some millions of Sahajdhari Sikhs from the franchise.