This document offers step by step ways to establish a GSA in your school. It includes activities, games, resources, information and tips for success.
PrideNet: Creating and Supporting a Gay/Straight Alliance
GSA Network: Trans and Queer Youth Uniting for Racial and Gender Justice
This site has a lot of great information regarding Gay Straight Alliances. It offers practical advice for created, maintaining and promoting GSAs in your school. (It is American site, geared for high schools but the information is definitely applicable to elementary.)
GSA Network: Trans and Queer Youth Uniting for Racial and Gender Justice