If the letter 'G' always follows a three digit number, try...
=MID(A1,FIND("G ",A1&" ")-3,3)
If the number is always 3 digits in length, try...
=MID(A1,MIN(SEARCH({0,1,2,3,4,5,6,7,8,9},A1&"0123456789",IF(ISNUMBER(SEARCH(" x ",A1)),SEARCH(" x ",A1),1))),3)
Otherwise, try...
=LOOKUP(9.99999999999999E+307,--MID(A1,MIN(SEARCH({0,1,2,3,4,5,6,7,8,9},A1&"0123456789",IF(ISNUMBER(SEARCH(" x ",A1)),SEARCH(" x ",A1),1))),ROW($A$1:INDEX(A:A,LEN($A$1)))))
Hope this helps!