Sometimes described as a "chemical brother" of gaba (gamma-aminobutyric acid), which amino acid neurotransmitter acts in an excitatory manner?

excitatory amino acids are the amino acids helps in transformation of neurotransmitters or it helps in transmission of synapsis rapidly in brain of mammal. EAA known to be neurotransmitters for Central nervous system.

Excitatory amino acids count may vary from 50’s to 100’s. They are mostly composed of non-protein- amino acids obtained from algae or fungi.

The possible EAAs are Glutamate (Glu) and Aspartate which act as excitatory neurotransmitters in the brain. They get released from neurons where they induce excitation via metabotropic Glu receptors.  

Both glutamate and aspartate having excitatory effect on neurotransmission whereas Gama-amino butyric acid having inhibitory effect on neurotransmission.

Thus, the statement ‘neurotransmitters are chemical brother of gaba’ is indicating the complementary effect of each other.