Even though it's a mouthful, we are
"Gustavus Adolphus College"
First reference to the College in text or body copy will always be the full name. "Gustavus," "Gustavus Adolphus," and "the College" (but never "GAC") are for second references, internal publications, or informal usage.
Display and image materials developed in conjunction with designers and marketing consultants and approved for official use (e.g., admission brochures, catalog covers, the College's homepage, etc.) may be exempted from this standard, but text copy in those materials should follow the rule.