In Giga3 (all versions)
On the far right hand side you will find the Halt/Start Audio Engine Button. ( it looks like a GEAR with a Green arrow)
On the farther right hand side you will find the Reset Button.
(Green Dot)
On the right of the Button there is a down arrow.
Clicking on this arrow will let you configure Reset to control any or all of these Giga Engines to reset:
Instruments ( This will clear all loaded instruments.)
Hardware
Mixer ( Will "zero" all mixer setting to default)
User Interface (Will clear list of loaded intruments)
Load Default Performance Settings
All (this will reset all above)

For instanly stopping sound (like for live on a stuck midi note:
use the Halt/Stop Audio engine (gear shape button)