Conversion between GB and MB (and KB)

You need to use 'better' units:

UnitConvert[Quantity[1, "GiB"], "MiB"]

Quantity[1024, "Mebibytes"]

See explanation and definitions in https://en.wikipedia.org/wiki/Gigabyte

Like the first answer said, you mean to use "GiB" and "MiB", the standard binary prefixes. These were set in place in the 90s to deal with this exact ambiguity, since everywhere but computing mega- and giga- referred to the standard metric prefixes based on powers of ten. Of course, powers of two are much more useful to us, so the new prefixes gibi- (1 GiB = 1024 MiB), mebi- (1 MiB = 1024 KiB), and so on were introduced.

You'll still see MB and GB used all over the place when people really mean MiB and GiB, though.

source: https://en.wikipedia.org/wiki/Binary_prefix