It will be helpful to know exactly which instrument, and which version of GPO you are working with.

Note that "Notation" variants of Garritan instruments do not show the keyswitches visually on the virtual keyboard of the plugin.  They are mapped starting with MIDI Note 0, which is beyond the range of this visual representation.  You should however, be able to see the key-switch 'name' toggle in the Notation versions. 

In contrast, the "Standard" variations of Garritan Instruments that include key-switches typically move them up to just beneath the normal playing range of an instrument, and thus are visible as off colored keys (Yellow or Red I think) in ARIA.

As for sustain...many instruments (such as strings and winds) in GPO do not use 'sustain' in a traditional way.  Instead CC64 enacts a special 'legato' mode.  What this means, is when CC64 is engaged, subsequent notes are 'crossfaded' a bit to emulate true legato articulations, and automated amounts of 'portamento' can optionally be added via other CC events.  GPO also offers a special 'auto legato' (Can be engaged/disengaged with CC102 events) mode which can put an instrument into 'mono' mode, and will apply these effects when notes 'overlap' a bit in duration.

With the new Orchestral Strings in GPO5, CC64 and CC68 seems to have some additional rules.  If you use 'Notation' versions of an instrument, CC64 can sustain them, while CC68 does the legato mode.  If you use the "Standard" variant of these new string instruments, then CC64 works 'legato' mode, while CC68 doesn't seem to do anything.

When working with instruments that do have proper sustain pedals (Pianos, Organs, etc.), then CC64 should work as you'd expect a sustain pedal to work.