For Gyn and Obs, include 2 custom GPTs:

Custom GPT 1: 

Includes Gyn and Obs textbooks + Green-top Guidelines + SLCOG Guidelines.
After you ask a question, this GPT refers to the included textbooks as well as all Green-top Guidelines and SLCOG Guidelines to provide an answer.

Custom GPT 2: 

Includes only Green-top Guidelines + SLCOG Guidelines.

After you ask a question, this GPT refers only to all Green-top Guidelines and SLCOG Guidelines to provide an answer.