Hall measurements have been performed as a function of hydrostatic pressure and temperature to study the effect of deep donor levels on the electron concentration and mobility in bulk GaAs heavily doped with germanium. For the first time, the coexistence of the metastable DX centre and the nonmetastable localized deep A1 level has been unambiguously observed in GaAs. Furthermore, the authors have determined the energy positions and pressure dependences of both resonant levels (0.066 eV and -6.9 meV kbar^-1 for the A1 level and 0.105 eV and -10 meV kbar^-1 for the DX centre) and the energy barrier for electron emission from the DX centre (0.285 eV). The latter appeared to be pressure independent. From the mobility behaviour during depopulation of DX centres, an elegant proof for the negative-U character of DX centres and the positive-U character of deep A1 levels has been obtained.