Based on the commercial silicon photonics (SiPh) process platform, a flat 3 dB bandwidth of 80 GHz germanium-silicon (Ge-Si) photodetector (PD) is experimentally demonstrated at a photocurrent of 0.8 mA. This outstanding bandwidth performance is achieved by using the gain peaking technique. It permits an 95% improvement in bandwidth without sacrificing responsivity and undesired effects. The peaked Ge-Si PD shows the external responsivity of 0.5 A/W and internal responsivity of 1.0 A/W at a wavelength of 1550 nm under -4 V bias voltage. The high-speed large signal reception capability of the peaked PD is comprehensively explored. Under the same transmitter state, the transmitter dispersion eye closure quaternary (TDECQ) penalties of the 60 and 90 Gbaud four-level pulse amplitude modulation (PAM-4) eye diagrams are about 2.33 and 2.76 dB, 1.68 and 2.45 dB for the un-peaked and peaked Ge-Si PD, respectively. When the reception speed increase to 100 and 120 Gbaud PAM-4, the TDECQ penalties are approximatively 2.53 and 3.99 dB. However, for the un-peaked PD, its TDECQ penalties cannot be calculated by oscilloscope. We also measure the bit error rate (BER) performances of the un-peaked and peaked Ge-Si PDs under different speed and optical power. For the peaked PD, the eye diagrams quality of 156 Gbit/s nonreturn-to-zero (NRZ), 145 Gbaud PAM-4, and 140 Gbaud eight-level pulse amplitude modulation (PAM-8) are as good as the 70 GHz Finisar PD. To the best of our knowledge, we report for the first-time a peaked Ge-Si PD operating at 420 Gbit/s per lane in an intensity modulation direct-detection (IM/DD) system. It might be also a potential solution to support the 800 G coherent optical receivers.