Catalyzes the cofactor-independent reversible oxidation of gamma-hydroxybutyrate (GHB) to succinic semialdehyde (SSA) coupled to reduction of 2-ketoglutarate (2-KG) to D-2-hydroxyglutarate (D-2-HG). D,L-3-hydroxyisobutyrate and L-3-hydroxybutyrate (L-3-OHB) are also substrates for HOT with 10-fold lower activities.