The Global Release Identifier (GRid) is a system for uniquely identifying Releases of music over electronic networks (that is, online stores where you can buy music as digital files). As that it can be seen as the equivalent of the BarCode (or more correctly the GTIN) as found on physical releases of music. Like the ISRC (a code for identifying single recordings as found on releases) it was developed by the IFPI but it does not appear to be a standard of the ISO.
MusicBrainz doesn't currently store GRids. In the future the barcode field of Release Events could be generalized to also accept GRids (but one would need a discriminator to state which code system is used).