It was very common for individuals to create "bibles" for classes they'd taken. Typically, a bible would include the person's notes taken during lecture and recitation, handouts, problem sets and their solutions, and exams and their solutions. Problem sets and exams changed often enough that you couldn't just copy the answers from last semester's bible onto this semester's problem set.
Some fraternities and sororities kept everyone's bibles (sometimes going back decades) in a central location in the house, and any brother/sister taking that course could refer to them. Some non-greek living groups kept centralized libraries, too - for example, I lived in a dorm with several floors, and my floor kept a library. So GLO members didn't really have a big advantage over non-GLO members. (As a member of an unhoused sorority, I had access to both my sisters' bibles and my floor's library.
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People often referred back to their own bibles from previous courses, too. Say course X was a prerequisite to course Y - it was often useful to have your bible from course X handy when working on problem sets for course Y.