Form Gran Colombia is the decision that forms 
Gran Colombia. It can be taken by any nation with the North Andean culture (which means that it is actually not a cultural union). Those are 
Colombia, 
Venezuela, 
Ecuador and 
Panama.
It follows the requirements of the usual formation decisions. The forming nation needs to have 45 prestige and own/sphere all the Gran Colombian cores. It also requires the forming nation to be at peace.