How could a genetically modified creature survive with only partial brain function? Theses are large (8'8) genetically modified humanoids. The brain is also larger. They are also surgically augmented. The reason for this is that i want to know if something like this could survive a gunshot through the head. It only needs to survive 1-2 hours without surgical intervention. They need to retain a human level of intelligence or at least regain it after a up to a few months. They have near future medical technologies to help with this.
The human brain is very complex, and if seems that every group of neurons is very specialized and unique. When we lose a substantial part of our brains, we recover not through neural backups, but due to other areas adapting to cover for the missing gray matter through plasticity.
An enlarged brain as you propose could introduce redundancy similar to what is done in some computer systems. Losing a part of your brain would not cause as much issue if you have an exact copy already plugged in.
Whether a shot is survivable depends on other things as well - you could die from blood loss, for example - but staying functional enough to walk to a hospital would definitely increase your odds of survival.
There is the famous case study of Phineas Gage. An industrial accident shot an iron rod through his head, in through the chin and out the top. He survived for 12 years and was able to talk and walk, though with major brain damage, seizures and personality change. This was with 1800s medical care. https://en.wikipedia.org/wiki/Phineas_Gage
The brain is not in the head.
It is really a pretty terrible place to keep a brain, and in unmodified animals the brain is there for evolutionary reasons. In these humanoids the brain is within the chest cavity, next to the heart and is heavily armored before and behind. Sensory apparatus (most of it) is still on the head as is the start of the digestive system and respiratory tract. The head still looks like a head. The cavity inside the skull has been repurposed for other functions.
A hit to the head could blind and deafen this creature. Serious damage could compromise its ability to breathe but the creature knows workaround for this to clear the airway. Bleeding from head trauma will not be as much of an issue as the carotid arteries are no longer in the neck. The creature might be confused because it has been badly hurt. But the brain will be safe from damage above the collarbones.