This will be determined by the variety that is being planted. Larger varieties would be spaced 2 to 3 feet between the plants and from the wall. 
Smaller varieties might be 1 to 2 feet. Air circulation is important for this plant so following what the planting tag instructions say is recommended for Gardenias.  Here is a link to an article we did on Gardenias. https://www.bloomingsecrets.com/blog/stop-and-smell-the-gardenias