[Galadriel55]

I think Gandalf had no fixed plan on purpose. If you make a plan from the outset, you tend to try to stick to it even though circumstances change, and the result might not be the best. So some bits, like Moria (and, had Gandalf lived to see the day, Parth Galen, and Henneth Annun, and the Crossings, and etc), had to be decided on the spot.

Gandalf made sure he knew his options. That's what the scouts from Rivendell were for. He knew which ways were open and which were not.

Some parts, like the getting-into-Mordor part, he had no solution for, so he could not have made a plan at that time, or really at any time. He probably thought to worry about those mountains when he got to them. He would have to think of it on the spot. And sometimes it's not that bad of an idea - once you see something firsthand you can try things and do things and see potential solutions, something you can't accomplish from a distance.

Mostly, though, I think his goal was to keep the "plan" as flexible as it could be. Like none of the Fellowship bound themselves to the Quest with oaths, Gandalf did not bind the Quest to a plan, since no one knows where it would lead.