Gain 1.
This is because of Chlorine's placement on the periodic table. Chlorine is a Halogen, thus being located in group 17 or 7A. This means it is one of the closest elements to being a Noble Gas or group 18/8A.
Chlorine has 7 valence electrons, and in order for it to become a Noble Gas, it needs 8. Cl- is typically the symbol used to represent this as Chlorine needs to gain an electrons instead of lose; once Chlorine gains the electron, it will be stable.
This also means that Chlorine has a high ionization energy or, in simpler terms, it would be difficult to remove an electron from Chlorine.
Hope this helps!
