I think there are a number of reason why GPO can sound as good as it does without the help of additional layers, wheteher controlled by velocity or round robin. One reason is that most of the instruments were recorded with vibrato, which of course is a pretty common articulation played by many musicians when playing the orchestral genre. Vibrato is not always desirable which is why many solo instruments do not have vibrato. There are many choices.
In the older (Kontakt) versions and the newer ARIA player by Garritan, there are two controls that can be controlled via midi CC. The two controls are labeled VAR 1 and VAR 2. If a phrase has many repeated notes in a row, the dreaded machine gun affect would be heard. This is minimized significantly with the use of some VAR 1 and VAR 2. Many people overuse these controls, but with a low setting, GPO can repeat the exact sample but with different tuning and timbre.
VAR 1 randomely alters the tuning each time GPO reproduces a new note, and VAR 2 randomely alters the equalization (tone).
I would never recommend a setting over 9 O'Clock.
GPO was designed when computers didn't have the processing power or memory of today's machines. It was designed so a whole orchestra can be loaded into 2 GB of RAM. This is impossible to do when more velocity layers are added. Also, the library was designed with laptops in mind.
Part of the reason GPO sounds as good as it does with mostly just one velocity layer is that CC1 controls both volume and timbre. There is some filtering going on when you lower the CC1.
Some instruments do have more than one velocity such as the piano and many of the percussion instruments.