Yes, this is true, goldfish and rosies are not the best choices for any reptile or fish to eat because of an enzyme called thiaminase. You can find lots of detailed information on that here:
http://www.thamnophis.com/caresheets/index.php?title=Thiamin,
and lots of information regarding proper garter snake care (including diet!) here!
http://www.thamnophis.com/caresheets/index.php?title=Garter_Snake_Care_Sheet

