[gorthaur_cruel]

In the Two Towers after Gandalf came back he said that he had become equal with the head nazgul.

He said that? The closests quotes I can think of is Aragorn saying, “The Dark Lord has Nine. But we have One, mightier than they: the White Rider. He has passed through the fire and the abyss, and they shall fear him.”. That suggests that Gandalf is stronger than all the Nazguls, which includes the Witch King. The other quote is when Gandalf replies "maybe", when Denethor says that the Witch King was a match for Gandalf.

I have questioned why is Gandulf not at least equal to Sauron since they are both Maia?

Not all humans are equally strong. Just because they're of the same race does not mean that they are equal. After all, the only difference between the Valar and the Maiar is power, since they were all Ainur to begin with. Gandalf's strengths lay in wisdom, rather than power.

Especially since Gandalf had a ruling ring and Sauron did not have his one ring.

The "Ruling Ring" refers to the One Ring. Gandalf had a "Ring of Power", not a ruling ring. However, Gandalf's Narya did not help whatsoever in battle, as is made clear. None of the Three gave any fighting advantages, but rather the power to preserve what is fair.

Also according to the Simillarion, Gandalf should be even better it says that Olorin was the chief Maia.

It says that he was the wisest. Do you think a...let's say, a wise old professor could kill a champion swordsman? I don't think so.

There's almost no doubt that Sauron was the more powerful in terms of strength between the two (despite the fact that Gandalf is my favorite character ). Tolkien wrote in one of his letters that if Gandalf decided to use the One Ring to fight Sauron 1v1, it would have been an even match. He also wrote that neither Galadriel or Elrond would've been able to fight Sauron 1v1 even if they used the One Ring.