G'day Bo,
Don't worry about the gradient so much at first. Use levels (mostly) and curves to adjust brightness etc. and color balance.
Use saturation to increase the color - you can use color select or other method to select stars only (to avoid increasing background color). There are tutorials on youtube. Don't increase the saturation too much in one go - do it in stages or you can over-saturate the color on some stars.
Once you are happy with the balance and color then you can work on the gradient (which isn't too bad).
Download a free software called Hasta La Vista Green - it is an add on for photoshop that helps get rid of unwanted green. In general there aren't many green hues in objects, except some planetary nebulae and comets and alternate color palettes.
If you spend a bit of time on this image you will find that it is really quite good - and you will learn a lot too.
These youtube videos are good:
http://www.youtube.com/channel/UCJ5b6pFHBGe66vsuSaXb-0A
Regards,
Tony.