Step up to the plate, act like an adult, and take responsibility. You can either marry the girl, let her do the single parenting thing and pay her child support, or help her place the baby for adoption.
Only marry her if you are financially stable and still have a good relationship with her. If she is dead-set on raising the baby herself, but marriage isn't a good option, then paying child support is the only responsible thing to do, but life for her will still be really hard.
Placing the baby for adoption might be the best option here. Your girlfriend will be able to choose which parents she wants to give her child to, based on pictures, letters, and profile info they provide. This option will give the baby the best shot possible at a loving, financially-stable home with a mother and father that love him/her. It will be tough for her to give the baby up, but you have to think of what is best for him. Many adoptive parents continue to send the birth mother letters and pictures after the child is adopted, so the birth mother can stay involved in her child's life (if she wants to).
Whatever the two of you decide, just take your time to make sure you are really making the right decision. You have 9 months- no need to make such a big decision overnight.