The way the card reads is unclear. Do GLC get the +1 bonus and the roll for outwit/perplex?

WILL: If this character already has the Green Lantern Corps keyword, modify all of its combat values except damage by +1. Otherwise, this character has the Green Lantern Corps keyword, can use Willpower, and when an opposing character targets it with Outwit or Perplex, roll a D6. On a result of 4-6, ignore that effect.

If the character has the keyword initially then it gets the +1 but not the other effects.

If a character with the Green Lantern Corps keyword is assigned the Green Lantern Ring they only get the modifier, not the last part. The last part is for characters that did not have the Green Lantern Corps keyword when the ring was assigned.

I'm not sure that last part is right... if they lose the keyword after being assigned the Ring, I believe they would gain the other bonuses (not the +1s, but the WP et al).

Although if they have the GLC keyword, they don't get the WP and OW/Perp protection and if they don't have the GLC keyword before being assigned the Ring, they don't get the +1s. Regardless of losing the keywords during the game, that is for sure.