Complete extent (4)
(Other definitions for span that I've seen before include "Arch across" , "Distance between two points in time or place" , "Measurement between tips of thumb, little finger" , "Distance or interval between two points" , "Bridge made of pans" .)
'complete extent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gamut that I've seen before include "Whole range (from A to Z)" , "Complete extent or range" , "Compass, scale" , "Complete range (of something)" , "Full range, compass" .)