Although I have read the following document (http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//005300000228000000) I am still totally unsure about how to enable a gml GetFeatureInfo response from ArcGIS Server 10.0 . Is this something on the client-side that needs to be implemented or something in my GetCapabilities Document?
If your GetCapabilities response has the following MIME type set then your client will be able to request GML.
The GetFeatureInfo request requires the following name=value parameter pair
EDIT (Following comments)
As ArGIS 10 no longer supports gml out-of-the-box, one solution is to create your own XSLT template in \Styles\WMS called featureinfo_application_vnd.ogc.gml.xsl
An alternative method would be to use EXOWS