1. gastrointestinal
2. general issue
3. gill(s)

adjective

1. Mil.
   1. galvanized iron [a GI can]: originally an abbreviation used in military paperwork
   2. now, government issue: designating clothing, equipment, etc. issued to military personnel
2. Informal
   1. of or characteristic of the U.S. armed forces or their personnel a GI haircut
   2. inclined to a strict observance of military regulations and customs a captain who is very GI
   3. of or for veterans of the U.S. armed forces

noun pl. GI's or GIs

1. gastrointestinal
2. general issue