How did Gally get into the control room?

In universe, the only indication we get is that Gally throws what we have to believe is an electronic "key" onto the floor as he stands in front Thomas and the others. He possibly could have received this by finding the smashed Grievers and extracting their key from them. Once the others had left the Glade, Gally would have had access to the maze map which the Runners were building and continually updating. This would give him an idea of how to find his way through the maze.

You are right though, as it doesn't explain how he would have known the correct series of sectors used as the pass code.

In the book it states that the 'Creators' could manipulate 'Gladers' to perform various tasks, one example is Chuck getting killed, he was manipulated to jump in front of Thomas, maybe the 'Creators' manipulated Gally and opened the 'Griever hole' for him, then he typed in the combination which he didn't know but was manipulated to do so

When the griever got smashed by the door that had closed when they put in the access code Gally must have gotten the key from that griever and if I'm not mistaken they had another one and Gally must have killed it but while he was trying to kill it, it stung him. And Gally must have seen the map and seen the code just lying around since the code was the order the maze moved in.

I think since Newt said that the key is "the same as we get in the supplies" "the box". That leaves me to believe that he took a key that had 7 on it. He knew the combination by being a keeper and the keepers keep the information about the maze. He also knew the 8 digit code. Then he entered the maze, in the process he gets stung.

My guess was that he learned the map in the hut. Got the key from a smashed griever, and learned the combination from the "serial number" on the key. As mentioned, the key had the same number as the supplies, which had the same amount of numbers as sectors, no? That's what I gather.

He got one of the keys from when all the doors opened. When they killed the Griever which almost killed Thomas, he grabbed a key and he got the code probably by eavesdropping when they first talked about the order and then he knew so then he got in. That is how Gally got into the facility.

As before he might have taken a key from a smashed griever thus why he got stung and learned the maze from the map but how did he know the combination for the door.

Simple answer: In the Runner hut there are boards filled with information about the maze of course Minho would have written the sequence down somewhere or he would have forgotten it and Gally would have stumbled across it.

Ok, this is a real scene, just came across it coincidentally by watching a music video, this clears up all of the mystery on how he got in... He didn't fight the grievers because he was suddenly being controlled by WCKD and where his wingmen are in the end is still a mystery.

He followed Thomas, Minho and the others look at around 3:22

Much clearer image:

WCKD used Gally as the last ever barrier from exiting the maze due to the fact that Thomas and Gally already had a tense rivalry.

I have realised, from reading different resources, that Gally didn't get in there on purpose, he was in fact controlled by the labs because of that microchip that is still in his brain.

I believe that he was manipulated by WICKED to go through the maze that he didn't know even the first few hundred metres of, and he was manipulated to run the maze. HE then found that there was dead Grievers laying around near the exit and slipped past while there was a 'quite' period, he was manipulated to pick up and amputate a 'key' which is the device to open the door to the 'Griever Hole' and he was forced to enter the sector sequence that he didn't already know by WICKED (they controlled his every movement) and he was controlled to got through the Griever Hole a little while after the other main Gladers went through.

He then was controlled in which direction he was going through the long corridor after the Griever Hole, he could not control anything that he was doing, he then pushed open the edit door to find the laboratory, where he found the other Gladers about to fully escape.

he was forcibly controlled to pick up a revolver from a dead scientist (or in the book it is a knife) and he didn't want to do any of this but he was being controlled to.

He demanded that everyone belongs in the maze and says some other stuff about belonging in the maze, then he (he already had the revolver poised) went to shoot (he was manipulated to do this as he did not want to kill or even harm any of his friends, they were his family) and Chuck was manipulated to jump in front of Thomas and was shot in the chest (or in the book he was stabbed by the knife) and in the movie Mihno threw a spear into Gally's heart, but in the book, Thomas jumps on top of him and nearly beats Gally to death, meanwhile Gally in his head was shouting:

"I'm sorry, I'm sorry I didn't mean to do any of this' and then he thought "maybe I do deserve this, maybe this is something that I have deserve for a long time"

and this is when it stops. Gally didn't like Thomas from day one, but every time when Gally tried to tell the others an idea, he is shot down in flames, he has tried for a very long time to get everyone out of the maze to safety and alive as he cared about everyone, but when Thomas came, Thomas was leading and it tore Gally in half and he snapped and decided that enough was enough.

WICKED found out about this and took it to their advantage and manipulated Gally to try to kill Thomas on their immediate escape, but Gally is the one who is shot down in flames.

This I believe is what happened and how he got in.