I wouldn't recommend getting the gravity ring as it would screw you over by making you an easy target to any of the dragon's attacks when it makes you move slowly while you're airborne. You also have to worry about jumping across the lava while you run from the dragon's fury. As he leisurely flies around the room, he will breathe out fireballs and lightning projectiles that will down you in a couple of shots, direct hit, or splash damage.
You're gonna have to take him down, but that's easier said than done. Who is the source of evil? The last enemy that you have to face to finish the game? It's a dragon that is guarding something important that is in the other room. The final level of Quake: Dissolution of Eternity, the Source of Evil.