  The magnetic properties of GdBaMn_{2}O_{5.0}, which exhibits charge ordering,
are studied from 2 to 400 K using single crystals. In a small magnetic field
applied along the easy axis, the magnetization M shows a temperature-induced
reversal which is sometimes found in ferrimagnets. In a large magnetic field,
on the other hand, a sharp change in the slope of M(T) coming from an unusual
turnabout of the magnetization of the Mn sublattices is observed. Those
observations are essentially explained by a molecular field theory which
highlights the role of delicate magnetic interactions between Gd^{3+} ions and
the antiferromagnetically coupled Mn^{2+}/Mn^{3+} sublattices.
