Gandalf in the 2nd Millennium of the Third Age

[Gordis]

The Witch-King certainly was among those who besieged Gandalf at Weathertop. The scene is described in Marquette MSS 4/2/36 "the Hunt for the Ring" published in RC p.167-8.

[On 2 October the four Black Riders who were sent ahead] assemble near Weathertop. [One] remains [while three] go on eastwards on or near Road. ... Oct. 3: Gandalf reaches Weathertop but does not overtake [Witch-king and other four Riders]; for they become aware of his approach as he overtakes them on Shadowfax, and withdraw into hiding beside the road. They close in behind. [The Witch-king] is both pleased and puzzled. For a while he had been in great fear, thinking that by some means Gandalf had got possession of the Ring and was now the Bearer; but as Gandalf passes he is aware that Gandalf has not got the Ring. What is he pursuing? He himself must be after the escaping Bearer; and it must therefore somehow have gone on far ahead. But Gandalf is a great power and enemy. He must be dealt with, and yet that needs great force.
[The five] follow Gandalf hotly to Weathertop. Since Gandalf halts there, [the Witch-King] suspects that that is a trysting place.
Gandalf is attacked by [the five plus the rider who had stayed near Weathertop] on Weathertop on night 3-4. Frodo and Aragorn see the light of the battle in the sky from their camp.
Oct. 4: Gandalf repulses the Nazgûl and escapes northwards at Sunrise, and follows the Hoarwell up towards the mountains. [Four Riders] are sent in pursuit (mainly because [the Witch-king] thinks it possible he may know of the whereabouts or course of the Bearer). But [the Witch-king and Khamûl] remain watching Weathertop. Thus they become aware of the approach of Frodo on Oct. 5. [The other three] return from East.

Now to the quote given by Ibrin("But our trial of strength is not yet come." ).
I think we must distinguish between the passing confrontations the WK and Gandalf have had before and the real "trial of strength", a fight to death that was about to happen at the Gates of Minas Tirith.

Gandalf the Grey had limitations imposed by the Valar :

[the Wizards] came out of the Far West and were messengers sent to contest the power of Sauron, and to unite all those who had the will to resist him; but they were forbidden to match his power with power, or to seek to dominate Elves or Men by force and fear.- TY, LOTR

So to go to Carn-Dum, find its King and try to kill him in a direct duel would have been strongly against the rules. All Gandal'f did at Weathertop and likely in similar situations before, was defend himself, when attacked, and then withdraw at the first opportunity. It was never a real "trial of strength".

In the same, almost anonymous, way Gandalf took part in battles. For instance in the battle of the Five Armies Gandalf was even wounded in the arm (see "the Hobbit"), but nothing about his part in the battle found its way into the "Tale of Years". He was there, but he was not in command, he displayed no supernatural powers, confronted no enemy commanders. This example shows that Gandalf may well have been say in the Battle of Fornost or in the battle of the Camps etc, without it being mentioned in the chronicles. Gandalf the Grey had been a self-effacing guy, always there somewhere in the background.

But that modus operendi changes when Gandalf turns the White. To start with, he shows his power to Aragorn, Legolas and Gimli, a thing that G the Grey had never done before. He assumes the command of the defense of Minas Tirith; in an open display of power Gandalf chases the nazgul attacking Faramir, and, finally, he openly challenges the enemy commander, the Lord of the Nazgul, the right hand of Sauron.

To me it seems that most of the limitations originally imposed on Gandalf the Grey by the Valar were lifted by Eru, when Gandalf was sent back. It is never told directly, IIRC, but it seems to be the case.