When an ion gains electrons What is the charge?

Negative(-)

The halogen atoms , e.g. chlorine, have electron space in their outer most electron energy shell to accept one more electron . This is known as 'Electron Affinity'. (NOT ionisation).

An electron is deemed to be of negative charge.

So the chlorine atom atom accepting one more electron, gains ONE(1) negative(-) charge.

Formulaically this is shown as

Cl(g) + e^(-) = Cl^(-)

Whereupon the chlorine atom is now referred to as the 'Chloride (an)ion '.

(NOT Chlorine ion).

NB The other halogen elements behave in a similar manner.