  Ac-suseptibility measurements were performed in the metallic helimagnet
Gd_{1-x}Y_{x} alloy. A remarkable increase of the imaginary part of the
ac-suseptibility was observed in the temperature range of the helimagnetic
phase. Moreover, a strong nonlinearity of the magnetization was observed at
paramagnetic-helimagnetic transition temperature T_{N}. On the other hand,
These anomalous behavior were not observed in similar rare-earth helimagnets Ho
and Ho_{1-x}Y_{x}. It strongly suggests that the weak magnetic anisotropy of
the Gd-moments is responsible for the anomalous slow dynamics in the
helimagnetic phase and for the strong nonlinearity of the magnetization at
T_{N}. The slow dynamics may result from a chiral-domain motion, or a long-time
variation of the period of the helimagnetic structure.
