\G is an anchor, which is a boundary match. It matches the ending of the previous match. Within some flavors, such as EditPadPro, \G matches at the start of the match attempt, rather than at the end of the previous match result.
\G with Perl
Using \G with Perl allows for a single, non-backtracking, non-restarting pass. It is important to anchor Perl where the last match ended and tell it to search and replace forward until the end of the string.
Other Languages and \G
In Java, \G is retained and recalled by the matcher object, which is linked with a single regex and subject string.
.NET, Java, PCRE, Perl