get the url and parse the string. there are various ways of parsing the strings.
use string manipulation functions in the silk
get the position in the url where GID= starts omit the previous part and work with the remaining string find position of "&" if it is present in the string and omit the portion after "&" the remaining string will be what you wanted GID=1234.
AsFarAsIKnow, There's no particular fn other than String parsing....U can write a fn. like this
STRING URLS= Browser.Location.GetText()