There are exactly 1 million gigabytes in a petabyte.

1 petabyte (PB) = 1,000,000 gigabyte (GB)

There are similar units based on powers of two instead of powers of ten, but at these large sizes, they are very rarely used, if ever:

1 pebibyte (PiB) = 1,048,576 gibibytes (GiB).

A terabyte is a trillion (short scale) bytes, 1012 bytes, or 1000 gigabytes.

The four larger official prefixes are:

petabyte = 1015 bytes = 1000 terabytes

exabyte = 1018 bytes

zettabyte = 1021 bytes

yottabyte = 1024 bytes

For values based on 1024 rather than 1000:

tebibyte = 240 bytes = 1024 gibibytes (giga-binary-bytes)

pebibyte = 250 bytes

exbibyte = 260 bytes

zebibyte = 270 bytes

yobibyte = 280 bytes