[lathspell]

Yes, Gandalf did indeed venture into Dol Guldur once. I don't have the books with me at te moment, so I can't tell you te exact date, but it was in the time that Sauron was still in Mirkwood and known as the Necromancer (as they also call him in the Hobbit). I have often wondered wether that wasn't what Gandalf had been doing when he left Bilbo and the dwarves to do something important for the White Council.

Anyway, the White Council was ofcourse aware of something evil lurking in Dol Guldur, and they suspected it was Sauron. Gandalf ventured into the fortress to find out if it really was Sauron. It was! Gandalf got back out, fled to Lothlorien and consulted the council. The White Council then went to Dol Guldur to fight Sauron, but he knew of their plans and had already commanded his Nazgul to make Barad-dur ready for his coming. Thus, the White Council came to Dol Guldur and found no Sauron there, for he was already in Mordor.
However, Gandalf says in the Unfinished Tales (in his own version of the Hobbit as he tells it to Frodo) that he believes that if Sauron wasn't forced to move to Mordor, he would probably have attacked the North first, thus wiping out Lothlorien and maybe Rivendell. And... would the dwarves have succeeded in killing Smaug, probably them as well.

I don't know if you know the game MERP (Middle-Earth Role Play), but in their version of the book 'Dol Guldur', there's the story as they think it happened.

greetings,
lathspell