No other way ?

[Legate of Amon Lanc]

Originally Posted by The Might

there could also be possible that he did think of Cirith Ungol himself, but only if he was there to help the Hobbits, and not if they would have been left alone with Gollum to lead them

Don't think so. Gandalf acts like this after hearing Faramir's news:

'Just a fool's hope, as I have been told. And when I heard of Cirith Ungol–––' He broke off and strode to the window as if his eyes could pierce the night in the East. 'Cirith Ungol!' he muttered. 'Why that way, I wonder?'

Gandalf seems rather, let's say, concerned about Cirith Ungol. Had he been a more nervous person, he'd probably say in this place something like "Oh no! Cirith Ungol! How could that fool of a Baggins even think about it!" And ending with screams of "he doomed us all, he doomed us all" he'd fall off of the balcony.

Back to reality, the possibility of taking some secret pass through the mountains seems logical, but I still can't help to doubt that Gandalf had known Mordor so well that he could come up with such a way, and as I said before, he couldn't have known if Sauron did not close that road since reocuppying Mordor (because if Gandalf even was at the borders of Mordor, I hardly believe that he did this when the Dark Lord was back there again). I remember Gandalf saying that "he visited only the older and lesser hold of the Enemy", Dol Guldur - and Aragorn seemed rather unnerved just from wandering the valley of Morgul and near the Black Gate, I doubt Gandalf ever did much more.
Maybe that idea of Orc disguise is not as bad - after all, it is a typical "heroic" way to get into the land of enemy (Beren, Finrod, Lúthien) and Frodo himself uses it in the end. I can imagine Gandalf metamorphing into a werewolf, Aragorn and Boromir masking as Uruk or Easterling warriors, Gimli as a strong Orc warrior, the hobbits becoming simple snagas (Phrodosh, Sham, Moriadog and Püpphan), and Legolas taking on a form of shadow-covered creature of dark. Then they go right through the Black Gate and straight to Mount Doom, where the last events take place.
This is the best idea I can come up with at the moment.