We demonstrate the generation of broadband THz pulses by optical rectification in GaP waveguides pumped by high power Yb-doped fiber amplifiers. The dispersion of the GaP emitter can be controlled via the geometry of the waveguide; the peak frequency of the emitted THz radiation is tuned by varying the waveguide cross-section. Most importantly, the use of a waveguide for the THz emission increases the coherent buildup length of the THz pulses and offers scalability to higher power; this was investigated by pumping a GaP waveguide emitter with a high power Yb-doped fiber laser system. A 25-MHz-repetition-rate pulse train of THz radiation with 120 muW average power was achieved using 14 W optical power, which represents the highest average power for a broadband THz source pumped by fiber lasers to date.