Allows the formation of correctly charged Gln-tRNA(Gln) through the transamidation of misacylated Glu-tRNA(Gln) in chloroplasts and mitochondria. The reaction takes place in the presence of glutamine and ATP through an activated gamma-phospho-Glu-tRNA(Gln). ATP + H2O + L-glutamine + L-glutamyl-tRNA(Gln) = ADP + H(+) + L-glutamate + L-glutaminyl-tRNA(Gln) + phosphate Subunit of the heterotrimeric GatCAB amidotransferase (AdT) complex, composed of A, B and C subunits. This protein may be expected to contain an N-terminal transit peptide but none has been predicted. Belongs to the amidase family. GatA subfamily.