[Legate of Amon Lanc]

I think there is no trouble in this. And I must say I agree with Morwen and do not see what case you are trying to make out of it. Gandalf merely tells a story, and I think that Gandalf's scaring of Gollum was nothing much more than what he showed to Bilbo when he wanted to keep the Ring. You must consider that Gandalf never forced anyone to do anything. So, though the fate of Middle-earth depended on it, Gandalf probably wouldn't force Gollum to tell the story by violence. The same way you may ask whether he would force Bilbo by violence, to leave the Ring to Frodo. Both Bilbo&Gollum, fortunately for them, chose to do what Gandalf asked of them before he did more than only demonstrating his power (Gandalf says he only scared Gollum, so I think this is what he actually did). And one can also think of what would've happened if they didn't: for example in Bilbo's case, there was no other choice than that Bilbo would totally harden against Gandalf, and put the Ring on, and flee into the night (because he would consider Gandalf his enemy and seeing the power he couldn't face, he'd flee). Gollum was not armed in any way, he could only desperate try to escape (the Elves possibly knocking him out, if not killing him when he'd jump on them, and on Gandalf's intervention locking him up probably for eternity - Gandalf might from time to time come to check whether Gollum would tell anything, but he probably wouldn't - I can quite imagine him sitting in the cell forever, unless some time mysteriously escaping).

Needless to say, the information Gandalf wanted to get out of Gollum wasn't as important; if he couldn't get confirmation of his fears here, he would probably go and ask Saruman, or if he felt reluctant, to check Bag End. As I said earlier.

Concerning Gandalf's character, you might confirm it also at how Gandalf behaved to Saruman, or to Gríma Wormtongue - people who were his enemies and he argued about something with him. Gandalf never used force in the "material" way (not even shooting lightnings on them or something like that).

And only as a bonus. Similar situation of the same moral question we have in the moment when Isildur chose to take the Ring, and both Elrond and Círdan chose not to push on him. This was at least of the same importance as those moments mentioned above, if not greater. But there was no "throw it in the fire or...!", not even forcing by words from what Elrond says (he speaks only of a "counsel"). And I think it was not fear of being killed by Isildur that lead Elrond and Círdan to choose not to force him.