Onix isn't convinced. I understand that. So, here is the justification of why you cannot hack Gaia. I'm going to draw parallels with today's technology and point out where they are very different.
What Gaia is
Gaia is three things. It is a load of energy patterns washing around around in the third medium. It is a piece of hardware that can access the third medium (Gaia terminal) and it is a language to interpret the energy patterns in the third medium (Gaia II). Together, they make Gaia. In C21, the third medium is 'the internet', your hardware is your computer and the language is Assembler (what your PC/Mac/whatever runs on).
Data retrieval
In Gaia, everything you see - the walls, the floor, the sun, the planets, the stars, the people moving around, the news articles, the messages sent between lovers, everything - is represented by an energy pattern. That data is swilling around inside the third medium and is plucked out by the Gaia terminal. The Gaia terminal is written by Star Sci. It cannot be changed - it is a hardcoded piece of hardware. In C21 terms, that is the processor inside your computer having the operating system embedded on it. You can't uninstall, you can't change. Can't upgrade. The terminal reads and writes to the third medium, it can't be changed.
I can make my own terminal?
You can but you won't be able to access Gaia. Inside every terminal is an encryption system (which is a volatile energy pattern) that encodes the messages going into the third medium and decodes them coming out. The encryption pattern is constantly changing. If you made your own terminal, you would need to get hold of this energy pattern to do it.
I can steal the energy pattern from a Star Sci made terminal
Energy patterns can be volatile, the encryption pattern is not 'observable'. This means that as soon as you try and touch it, sense it, read it, copy it, move it or do anything with it, you change it. You force it into a state. Even for a moment. As soon as you do that, it will lose time synchronicity with all the encryption patterns and it will no longer be able to encrypt or decrypt the data floating around in the third medium. The only way to create this pattern is to do so from its source, which is closely guarded.
I can steal the thing that makes the energy patterns
You can try. You won't survive. You'll have the Fleet Elite after you - they have powered armour that Light Jumps and can cloak.
I can create a hacking probe that sits between the terminal and the third medium
You can't, the third medium is superimposed on top of reality, you are moving through the third medium all the time. The insides of the terminal contact Gaia without any signals leaving its very core.
I can hack Gaia II, the language that interprets Gaia on my terminal
Nope, that is hardcoded too. It hasn't changed in 1000 years.
So, what about the Undergaia?
The Gaia II language that builds the virtual world isn't perfect. It has some foibles, which allows the intelligent hacker to break into areas that should be secure. Using similar techniques, you can hide places where you shouldn't be able to - creating the Undergaia. But that is all in the virtual world of Gaia, not within the terminal. In C21 parlance, you can make programs running on your computer do horrible things but you still can't hack the hardcoded OS on your processor.
Star Sci aren't so bothered about the little flaws in Gaia II - they had to rush it out to give humanity a communication mechanism the Droids couldn't use.
I hope that clears it up.