Hi Diane,
I have figured out a way to have a dynamic parameters (whether it's a drop down or multi select widget) based on a selected value from another parameter in my WAB gp widget.
My suggested workflow:
1). On your GP widget UI, find the widget ID of the parameter you want to monitor using dijit.byId or registry.byId.
2). Create an event handler to watch for changes in value to this editor and capture the value it was changed to
3). Write the code to create the variables with your desired dynamic values based on what was selected in step 1 using if/then statements, getting results from a GP service, or whatever logic you need, etc
4). Find the widget ID of the parameter that needs to be dynamically changed based on the value selected in step 1.
5). Use registry.byId's set/reset method to update the values of the widget in step 4 dynamically with the variables you created.
Note: you may have to use dojo/ready and dojo/connect to make sure your widget is fully loaded.
Hope this helps!