What wonderful news. If they're happy, I hope you can be happy and I'm sorry you had to tell them it's not what you would have wanted. As you say, they are adults, and they are past the point of running things by you.
Are her parents both in the picture? So do you have to juggle names with them? Are there stepparents? If so, I'd be sure to make a list of things you like, perhaps in priority order, so they have more than one choice in case your first choice is "taken." If there's anything you truly hate, you can put that at the bottom of the list as a "not my favorite" but be sure you have at least 3 choices above it. My guess is, once the baby comes, you won't be the least bit worried about your name!
My husband and I are Nana and Grandpa. So were my parents. My husband's mother was Grammy. My grandparents were Grandma/Grandpa on one side, Nana/PopPop on the other. My neighbor's parents use Polish names. My college roommate, whose family is half Ecuadorian/Spanish speaking, uses "Buita" (pronounced BWEE-tah, a nickname for Abuelita/Little Grandma) and Papa for her husband. Her mother is Grandma to my roommate's (now grown) kids. My other neighbors (French extraction) are Memere and Pepere, "Mem" and "Pep" for short. Many Jewish friends use Bubbe and Zaide (Yiddish) or Saba and Savta (Hebrew) Greek cousins use YiaYia and Papou. If either side has an ethnic heritage to honor, you could consider that even if the names weren't used in your own family when you were growing up.
Mostly, I wouldn't worry now about making a choice. Take your time with this because the name is going to stick for decades! The baby won't be here for many months and won't be talking for months after that. A wise decision will make you happier, so don't rush. And do tell them you are glad you have the time to consider, and that you want to give her family plenty of leeway too. It will go a long way in your relationship with your son's girlfriend, believe me.