By Default, the ARIA Player itself does the following:

As per GM:  CC7 is channel volume.

As per GM:  CC10 is pan.

As per GM:  CC11 is 'expression volume' that is scaled relative to the channel volume.

Many Garritan instruments use the Mod Wheel's CC1 instead of key velocity to set the initial attack volume, and for notes that need expressive dynamic control over time.  CC1 is identical to CC11, and you can use either or both (I.E.  Set attack volume with CC11, and crescendo with CC1 to help maintain some separation in your DAW).  In my experience this is not a problem with most Modern Scoring Applications and tracking DAWs.  The setup allows the hosting software maximum expressive and dynamic control in real time.

So far for me, it's only a problem when trying to build a bridge to use Garritan Libraries with Muse Score (which cannot be set to maintain attack volume with a CC).  Finale, Sibelius, and Dorico all handle it with ease.  For MuseScore I opt for "Garritan Instant Orchestra" (which has a GM mode) instead of GPO4/5, or host ARIA in Bidule and build some logic to snoop key-velocity and insert a CC1 event before each note.

In my tracking DAW (CuBase Pro) I simply run 'logic editors' to quickly insert an event before each note that is equal to either the pitch, or the velocity of the note (depending on the nature of the instrument, and the dynamic effect I want) and then fine tune as needed on a note by note basis from there in CC lanes.  Logic Editor make quick work of roughing in the overall dynamic feel for the part.  I can quickly and easily make a passage grow a little louder as pitch rises, and softer as pitch lowers.  I get 'full control' from the host sequence, in regards of choosing the best sample layer at the best volume....not always the case with GM style instruments that handle more based on key-velocity and static ADSR envelopes in the synth engine.

Quite a few Garritan instruments (winds, brass, bowed strings) use CC1 (Mod wheel), CC2 (Wind controller), or CC11 (Expression) for general dynamics relative to your Master Channel Volume (CC7).

Velocity for most of these non percussive instruments emulates aggressiveness/pressure on articulations independently of the expressive volume.  The idea is that you might want an intense amount of 'pressure/aggressiveness' with a shorter attack style even though you are playing pp.  You might want less aggressive velocity attacks even though you are playing ff.  In reality, string, wind, and brass players almost always have some Fz or Sfz to their attacks.  Things like staccato or accent markings mean something....and apps like Finale, Sibelius, Dorico, etc...do take advantage of CC1 or CC11 to make these things happen.  Meanwhile, one can also maintain a different velocity for selecting preferred sample layers or filtering/eq envelopes in the synth engine.

Some of the instruments can also do LFO generated vibrato (not all can do this, I think it's mostly JABB that has some LFO supporting instruments).  In those cases after-touch is used to trigger Aria's LFO opcodes.  If you do not have a keyboard-controller or DAW that can send channel after-touch events, you can add some other CC in the instrument's SFZ file to control the LFO. 

Here is one thread for an example of changing the LFO vibrato trigger from after-touch to something else:

https://makemusic.zendesk.com/hc/en-us/community/posts/115000488188-Using-After-Touch-during-performance-on-my-midi-Keyboard

Percussive sounds (Pianos, Drums, Mallets, etc.) are typically more attuned to do more to relative dynamics based on key velocity.  Some allow you some choices.

Organs and synths can vary between the approaches above, but in general, acoustic organs use the CC1/CC11 method, while various electronic instruments might have more velocity based dynamic control.

Users can dig into ARIA instrument patches and make changes.  To some degree you can also dig into the UI that shows up for an instrument in ARIA to make changes there (Change the CC by which a knob can be automated by). 

Each Garritan instrument is built using an SFZ text file, which you can back up, then open in your favorite text editor.  One can find a listing of SFZ 2.0 opcodes here:  http://drealm.info/sfz/plj-sfz.xhtml

If you do not like the default Garritan setup, good luck finding advanced/expressive sample libraries that are 100% GM compliant.  Separating sample layers via velocity while setting attack volume with CC1 or CC11 is a pretty run of the mill methodology in orchestral sample libraries these days.  All of the leading scoring apps but MuseScore support it.  A few libraries do exist that have options to chose between velocity or CC based attacks, but few are as inexpensive and expansive (sheer number of instruments included).

So yes, there are quite a few really nice orchestral libraries out there, and some do offer more obvious flexibility in how instruments can accept and interpret dynamics, but you probably will not be finding them for less than $200.

Garritan Instant Orchestra has a GM mode for each instrument.  This is more of a texture pack than a complete individualized orchestral library such as "Garritan Personal Orchestra.  Instead of loading up full sections on a player by player basis as you might with GPO, you get things like, "Silvery Winds" (flute sections),  Big Brass (again a section), etc.  You get some options for soloists, and a ton of pre-made 'orchestral effects'.

Your next options on the table that are likely to work in the full GM style are double or even quadruple in price, and when in GM mode, you loose a good 20% of your expressive control.  For instance, something like HALion 6, or Kontakt comes with a very nice slate of GM compatible sounds for all genera of music that work well via GM protocols right out of the box.  When played 'dry' they can come across as being a bit lame, but such engines also have scads of nice effects (reverbs, chorus, compressors, equalizers, etc.) included that make it all sound great together.  They also have expansive UIs where you can dig in and easily manipulate a patch to perform as you like.  You can even build your own instruments from the ground up (sampling as well).  You can also add more libraries (at additional costs) with such engines.

In the same price range as GPO5, you can find HALion Symphonic Orchestra.  That one does allow you to choose between velocity or CC style dynamic control on the attack.  It needs a dongle if you want to use it on more than one computer.  It doesn't have as many instruments and layers (individual players to choose from).  You get no harps or organs!  Some of the library is quite nice, but some of it (solo strings) is pretty terrible in my opinion.

SONiVOX has a Film Scoring series of plugins that are very GM centric.  They sound nice too!  They need a pretty beefy PC in my experience as they often spike my old AMD Phenom II x6 CPU.  You do not get a fraction of the number of instruments (Again, no harps or organs).  No solo strings at all.  It's also a good bit more expensive than GPO5.