Well I'm not familiar to GIMP, but when you want to create a glow effect like in a laser or something you create a bright spot ( white ) and a quite large colored, transparent spot with feathered edges below.
So create two layers and color your Eye white and then use a quite blured pen and make some thick outlines (colored ) aroud the white eye and make the layer with the color transparent or try and play with the transfer modes.
Thats how I would do it.