ismemberOf() returns true/false so you don't have to do the = "true" part.   And you don't use if/then in a condition.   The condition itself is the if part.   Just put the following in the condition script:

gs.getUser().isMemberOf("IT ServiceDesk");

I assume you have other conditions so you'll have to add them to that, for example...

gs.getUser().isMemberOf("IT ServiceDesk") && current.somefield == "somevalue" && etc....

EDIT: you may be right that the advanced condition script requires the full if/then...return syntax, there are places where this isn't necessary so I may be crossing my wires