Gan Israel (Gan Yisrael) is the network of Chabad summer camps for children, named for the Baal Shem Tov (Rabbi Yisrael). These letters are the Rebbe's messages to the campers and staff, mostly from the 1960s. See also Jewish outreach – Summer camps, youth groups and reaching Jewish children.
A model of Torah living. The Rebbe asked that time at camp be a model of Torah study, mitzvah observance and good conduct (IK 10439), that camp activities foster both physical and spiritual well-being (IK 10176), and that the campers succeed in Torah study and mitzvos (IK 9000). True success in every area, including health, comes through added dedication to Torah and mitzvos (IK 8861).
Special times. He noted the special significance of Gan Israel opening in a Hakhel year and on a meaningful date (IK 9341), and of the camp season during the Shemitah year (IK 9166).
Sources: the Rebbe's letters cited above.