One gigabyte is 1,000 megabytes under the SI definition used on this site, so a megabyte is 0.001 GB. If your operating system tells you something different, it is using 1,024 — and that disagreement is the defining quarrel of digital storage units.
About the units
A byte is eight bits, and the prefixes attached to it were borrowed from the metric system, where mega means 10⁶ and giga 10⁹. Computing then quietly redefined them: because memory addressing is binary, a "megabyte" of RAM was always 1,048,576 bytes, which is 2²⁰. Storage manufacturers kept the decimal meaning, operating systems adopted the binary one, and both called the result MB. In 1998 the IEC introduced separate binary prefixes — kibi, mebi, gibi — so that MiB means 1,048,576 bytes unambiguously and MB means exactly one million. Linux and macOS now follow the decimal convention for storage; Windows still reports binary values labelled with decimal prefixes.
The exact factor
Under the SI convention this page uses, the factor is exactly 1,000: 2,500 MB is 2.5 GB, and 0.75 GB is 750 MB. Under the binary convention it would be 1,024, a difference of 2.4 per cent at this step. The gap compounds with each prefix: 2.4 per cent at kilo, 4.9 at mega, 7.4 at giga and 10.0 at tera. That is why the discrepancy is invisible on a memory card and glaring on a hard drive.
Where you meet this conversion
File sizes, download quotas, email attachment limits and phone storage. Mobile data plans are sold in gigabytes and individual app downloads listed in megabytes, so estimating how much of a monthly allowance a download consumes runs through this factor constantly. Camera raw files, video bitrates and backup sizes all sit at the boundary between the two units, which is where the choice of convention starts to matter for whether something fits.