Acts as a component of the translation initiation factor 2B (eIF2B) complex, which catalyzes the exchange of GDP for GTP on eukaryotic initiation factor 2 (eIF2) gamma subunit. Its guanine nucleotide exchange factor activity is repressed when bound to eIF2 complex phosphorylated on the alpha subunit, thereby limiting the amount of methionyl-initiator methionine tRNA available to the ribosome and consequently global translation is repressed. Activated by the chemical integrated stress response (ISR) inhibitor ISRIB which stimulates guanine nucleotide exchange factor activity for both phosphorylated and unphosphorylated eIF2.