Other section strings like Violins and Viola in GPO are multi-timbral, but with the Cellos KS, I can only get one note to play at a time.  Do I have a wrong setting?  Or maybe cellos normally play only one note at a time?

I'm trying to find a cello patch in GPO that does what you say.  All of mine play multiple notes.  I'm using the GPO in Dimension Pro and have tried ever cello patch in the solo and section string.  all allowed multi for me.

You could check the following :

1) keyboard transposition settings on your controller.  one of my keyboards has a nasty habit of jumping up or down an octave.  Since the string sections have a very limited range they don't map across all 88 keys, an octave shift may mean the other notes you are playing are out of the mapped range.

2) Check the file name of the elements that are loaded (if using Dimension pro or le) the sample sfz file typically matches the file.  if not, maybe somehow a patched got corrupted or you saved over a preset

Cellos Ks mean that they are key switched, It maybe that the switch has latched to legato mode?

I'm using GPO with the Aria player, not dimension pro.  I checked the keyswitches and nothing is latched legato.  But I did check another project and Cello plays multitimbral no problem.  So something in my current project is turning on legato on for the Cello.

Guitartrek, on the control page of the Cellos KS check to see if the auto legato button is checked. Also use the midi editor and see if CC102 is in the mix. Both of them turn on/off legato.

I hope this helps and good luck – MarioD – another guitarist.

MarioD - Thank you!  I didn't even know that there was a separate control panel for each instrument.  The auto legatto was On!