  We have produced a soft x-ray supercontinuum with a pulse energy of
0.24-$\mu$J using high-order harmonics from a multiterawatt, 10-Hz, stable,
three-channel waveform synthesizer [Sci. Adv. eaay2802 (2020)]. We report here
an attosecond streaking scheme, which is designed for measuring
low-repetition-rate, high-energy, isolated attosecond pulses. The retrieved
pulse duration is 226 as, which conclusively demonstrates the realization of
1.1-gigawatt isolated attosecond pulses at the generation point. The
gigawatt-level peak power of this tabletop, isolated-attosecond-pulse light
source is equivalent to that of an x-ray free-electron laser in a large
facility.
