When two characters mirror each other, those characters are called foils of each other. The purpose of a foil is to emphasize certain characteristics.
Lear and Gloucester foil each other in a few ways. First, their families are nearly identical.
The issue of blindness also makes the characters foils of each other. Gloucester is blind to the truth while Lear becomes literally blind in the end of the play.
It can also be argued that both characters are defeatists, but you would have to find evidence for that.