  We report on successful synthesis under high pressure of a series of
polycrystalline GdFeAs O_{1-x}F_x high-Tc superconductors with different oxygen
deficiency x=0.12 - 0.16 and also with no fluorine. We have found that the
high-pressure synthesis technique is crucial for obtaining almost single-phase
superconducting materials: by synthesizing the same compounds with no pressure
in ampoules we obtained non-superconducting materials with an admixture of
incidental phases. Critical temperature for all the materials was in the range
40 to 53K. The temperature derivative of the critical field dHc2/dT is
remarkably high, indicating potentially high value of the second critical field
Hc2 ~ 130T.
