Since GABA is the main inhibitory transmitter in the brain, if levels rise high enough could it cause anxiolosis and eventually coma?
Do I understand? Great question. I think you are asking whether gamma-amino butyric acid can produce excitoxicity. This is a completely different function from synaptic inhibition, and it's been reported in some animal models recently especially in the immature hippocampus and conjecturally as a cause of the brain damage from repeated alcohol / sedative withdrawl. Check the scientific papers yourself.
High GABA levels. Not aware of any data that suggests it is possible to cause a dramatic imbalance in gaba to cause coma in live intact cns. Lethal overdoses from "gaba-ergic" medicines is extremely rare. Remember that (simplistically stated) gaba and glutamate (excitatory neurotransmitter) are in a dynamic balance the nervous system is incredibly designed!