We test IntelliGantt against Windows Terminal Server, but do not have a Citrix environment in house. However, we are working with a client right now to deploy on Xen 4.5 so we will have a definitive answer for Citrix soon. The intention is certainly 'Yes'.
From a licensing perspective, IntelliGantt is licensed per user. So if you deployed IntelliGantt to a Citrix (or Windows Terminal) Server, and had 10 users of IntelliGantt, then you would need 10 licenses.
Note that this is only for the Rich Client installation. If 9 of those users only needed access to SharePoint through the web browswer to view and update tasks, then only 1 Rich Client would be needed-- there is no CAL for SharePoint users.
Thanks for the question,