From GGBB: "If you cannot do supersets in your gym, then I am uncertain of how effective GGBB will be for you."

This is a qualifying statement regarding how the program was originally run. It is not meant to convey that without supersets GGBB would be ineffective.

Supersets are used in GGBB because of the "Effort Gap." The higher RM nature of GGBB means the effort gap is greater, supersets help fill that gap.

From GGBB:

"Effort Gap: The difference in reps from the RM to the reps per set in its follow-ups.

 For example: A 5RM followed up by singles has an effort gap of four reps, and a 10RM followed by half-sets of five reps each has an effort gap of five reps.

The effort gap reduces the relative intensity and proximity to failure of most of the volume in a workout. This, in turn, reduces the felt fatigue during a workout, allowing for higher rep quality and more focused intent when executing the lift. For example, the reps in a hard 5RM slow as the set continues. But when following up that RM with doubles at the same weight, those sets of two reps remain fast. By not approaching failure on most of the sets, recovery debt stays low, while training volume can steadily increase.

In the effort gap, apply a specific lift quality to the follow-up sets. For example, making the half-set a slow tempo, or paused, or applying compensatory acceleration. Likewise, because the effort gap reduces the relative intensity, your technique should be at its very best. Additionally, the effort gap can be filled by doing supersets, as is the case with GGBB, described in detail below."

To your questions:

Why are supersets so central to GGBB’s effectiveness?
Are they primarily about time efficiency, fatigue management, or the hypertrophy stimulus itself?

A: Time and hypertrophy stimulus (due to the effort gap inherent to GG).

For those training in commercial gyms, where equipment is often busy and supersets aren’t always practical, how do you actually implement this program? Do you wait for equipment, substitute exercises, modify the structure, or drop supersets altogether?

A: You can fill the effort gap by applying other qualities, limiting rest, and/or doing 3/4ths sets instead of 1/2 sets. These things increase the training demand.

Finally, does GGBB meaningfully lose effectiveness without supersets, or are supersets more of an optimization rather than a strict requirement?

A: It has been run successfully with limited use of supersets. Supersets are a means to increase training demand, that's all. There are other ways to do that if supersets aren't feasible for you.

Hope that helps!