Inhibits all catalytic activities of RNA polymerase (RNAP) by partially occluding its substrate-binding site and preventing NTP binding. Interacts with RNAP. Inhibitory activity is regulated via a pH-induced conformational change of the structure. At pH above 7, Gfh1 is in an inactive flipped orientation that prevents binding to RNAP. At lower pH, Gfh1 switches to an active orientation, which enables binding to RNAP and inhibitory activity. Belongs to the GreA/GreB family.