How can i get green lantern power?

You're correct. A Ring weapon and a Head weapon were both shown in Alpha footage of the game from E3 in either 2008 or 2009, but they never made it into the final game. The footage only showed them using ranged attacks, so my guess is that maybe they couldn't come up with a suitable set of melee options for those weapons and had to scrap them. Light was also originally blue in colour (but not a Blue Lantern power set) before ultimately making it into the game as green and yellow.

Wait, a head weapon?

Yeah, like shooting blasts from your head. You can see it a little bit in this video, starting at 30 seconds.

Thank you