[Eurytus]

Of course even if Gandalf had not been in Mordor it is likely that he would have had access to detailed maps of its layout, even if these maps were somewhat out of date.
After all, after the Last Alliance the men of Gondor actually built many of the structures in Mordor that Sauron would later use. Originally they were for the purpose of keeping a watch on Mordor.
It is likely that during this time the men of Gondor would have made maps of Mordor, both to be able to best assess the positioning of their towers and for the purpose of troop movements.