I imagine the over 1GHz in this case is the baseband that you are talking about.

ADS-B baseband is also around 1GHz, what they mean is once you down convert in the process of demodulating the signal you will get a signal that has bandwidth of 1.3MHz.

It's the width of the band that matters, not the carrier frequency. For example a band running from 1.21 GHz to 1.22 GHz has a 10 MHz wide band. A heterodyne tuner would use analog circuitry to output it as a 0 to 10 MHz baseband signal that can the be sampled by the ADC.

GPS bands are from 11 to just over 15 MHz, depending on the band, but decoding is extremely complex, so it's usually done with dedicated hardware, so a microcontroller only needs to read the handful of bytes that hardware outputs per update, and most receivers default to only a single update per second, and max out at tens per second.