GTDB uses C5 as middle C. The ISO standard is C4 which makes the lowest C on a piano C0. Some keyboards, including Yamaha, use C3 as their middle C which would make the lowest C on a piano C-1.
The middle C on GTDB displays as C5 however the frequency is around 261.63 Hz as is what middle C is supposed to be since the pitch standard was widely adopted in 1939.
The lowest C on a piano according to GTDB is C1.