GVT -- Too Much Volume!? Only Trunk / Legs, and Triceps, not Pecs, Biceps or Back Benefit from Doing 10 vs. 5 Sets

"Allegedly proven"? Yes, you read me right. In spite of the fact that it has been around for decades, Amirthalingam et al., the authors of a soon-to-be-published paper in the Journal of Strength & Conditioning Research claim that, "to date, no study has directly examined the training adaptations following GVT" (Amirthalingam. 2016). The purpose of Amirthalingam's latest study was thus to investigate the effect of a modified GVT intervention on muscular hypertrophy and strength.
Nineteen healthy males were randomly assigned to 6 weeks of 10 or 5 sets of 10 repetitions (or as many reps as possible before failing) for specific compound resistance exercises included in a split-routine performed 3 times per week (workout details in Table 1):

Table 1: Overview of the two workout programs (Amirthalingam. 2016).

Mind the fact that, as it is in the original GVT protocol, only the primary movers, the bench press, lat pulldown, leg press, dumbbell lunge, shoulder press and upright row were performed for 10, the auxiliary exercises for 4x10 reps. Before and after the training program, total and regional lean body mass, muscle thickness, and muscle strength were measured by DEXA and ultrasound, as well as standardized strength tests. 

As you can see in Figure 1, there were significant increases in lean body mass measures across the groups. However, a significantly greater increases in trunk (p = 0.043; ES = -0.21) and arm (p = 0.083; ES = -0.25) lean body mass favored the 5-SET and thus the lower volume group.

Figure 2: Relative changes in muscle thickness and strength (Amirthalingam. 2016).

Similar results were observed for muscular strength, with greater increases in the 5-SET group for bench press (p = 0.014; ES = -0.43) and lat pull-down (p = 0.003; ES = -0.54). That's surprising - at least in view of the results of the latest reviews of the effects training volume on muscle and strength gains - reviews that suggest that volume is the key to your gains.
References:

• Amirthalingam, et al. "Effects of a Modified German Volume Training Program on Muscular Hypertrophy and Strength." Journal of Strength & Conditioning Research: Post Acceptance: November 25, 2016 | doi: 10.1519/JSC.0000000000001747.