  One month after launching Gravitational wave high-energy Electromagnetic
Counterpart All-sky Monitor (GECAM), a bright thermonuclear X-ray burst from
4U~0614+09, was observed on January 24, 2021. We report the time-resolved
spectroscopy of the burst and a burst oscillation detection at 413 Hz with a
fractional amplitude 3.4\% (rms). This coincides with the burst oscillation
previously discovered with \textit{Swift}/BAT \citep{Strohmayer2008}, and
therefore confirms the spin frequency of this source. This burst is the
brightest one in the normal bursts (except the superburst) ever detected from
4U~0614+09, which leads to an upper limit of distance estimation as 3.1 kpc.
The folded light curve during the burst oscillation shows a multi-peak
structure, which is the first case observed during a single burst oscillation
in nonpulsating sources. The multi-peak profile could be due to additional
harmonics of the burst oscillation, which is corresponding to several
brighter/fainter spots at the stellar surface.
