Aron,
Have you not seen the GP samples like http://help.arcgis.com/en/webapi/flex/samples/index.html?sample=GP_Viewshed ?
Aron,
Just because you have the GP setup and working in desktop does not mean that you have all the parameters setup correctly for the GP to be published and usable as a service in server. There are some very specific step you have to take for a GP to be ready to publish to server. I cover the steps for start to finish in the thread.
http://forums.esri.com/Thread.asp?c=158&f=2421&t=285944&mc=7
What is the execution type? If it is "esriExecutionTypeAsynchronous" like this one, then you'll need to call submitJob instead of execute.
http://sampleserver4.arcgisonline.com/ArcGIS/rest/services/HomelandSecurity/Incident_Data_Extraction...
Reference:
http://help.arcgis.com/en/webapi/flex/apiref/com/esri/ags/tasks/Geoprocessor.html