  We revisit the hybrid inflation model within the framework of the Grand
Unified Theory (GUT), focusing on cases where the waterfall phase transition
extends over several e-foldings to dilute monopoles. Considering the stochastic
effects of quantum fluctuations, we demonstrate that the waterfall fields
(i.e., GUT Higgs) maintain a nonzero vacuum expectation value around the
waterfall phase transition. By accurately accounting for the number of degrees
of freedom of the GUT Higgs field, we establish that these fluctuations can
produce observable gravitational waves without leading to an overproduction of
primordial black holes. The amplitude of these gravitational waves is inversely
proportional to the degrees of freedom of the waterfall fields, thereby
providing a unique method to probe the representation of the GUT Higgs.
