I have taken the below quote from the SGPC website - I dunno about anyone else but it sounds to me as if the Rehat Maryada was created by the SGPC.
"The Shiromani Gurdwara Prabhandhak Committee had constituted a sub-committee comprising the undermentioned gentlemen for preparing a draft of code of conduct and conventions to enable it to determine and prescribe a proper set of conventions for gurdwaras (Sikh places of worship):
Giani Thakar Singh, Amritsar; Giani Sher Singh; Bhai Budh Singh; Akali Kaur Singh; Sant Sangat Singh of Kamaliya; Bhai Kahn Singh of Nabha; Sant Gulab Singh of Gholiya; Bhai Labh Singh, Granthi, Sri Harmandar Sahib; Bhai Hazura Singh of Hazur Sahib (or a representative of his); Pandit Basant Singh of Patiala; Bhai Vir Singh of Amritsar; Giani Hira Singh Dard; Bawa Harkishan Singh, Principal, Guru Nanak Khalsa College, Gujranwala; Bhai Trilochan Singh of Sur Singh, Distt. Lahore; Giani Hamir Singh of Amritsar; Pandit Kartar Singh of Dakha, Distt. Ludhiana; the Jathedar of Sri Akal Takhat; the Jathedar of Takht Sri Kesgarh Sahib, the Jathedar of the Takht Sri Patna Sahib; Prof. Ganga Singh; Prof. Jodh Singh; Sant Man Singh of Kankhal; justice Teja Singh; Bhai Randhir Singh; and Prof. Teja Singh (who was to be the convenor of the sub-committee).
The meetings of this sub-committee were held on Oct. 4 and 5, 1931, Jan. 3,1932, and Jan. 31, 1932 at the Akal Takht."