How can I get my GF to talk more like a grown-up?

Talk to her like a highly educated, sophisticated person, and hopefully she'll catch on and do likewise. Alternatively, if you don't mind being a little harsh about it, you could just ignore her when she talks like that, and when she asks you about it, tell her you don't understand baby-talk etc, but do be cautious, you don't want to hurt her feelings too much. The other thing you could do is come up with a code-word that you can say to remind her that she's talking silly and that you'd like her to just talk like a normal person, that way it's a quick way to send a message and you don't have to have a whole discussion about it all over again.

I am an adult and I sound like a little 5 year old girl. Many times when I answer the phone I am asked if my mommy is home. I cannot help this, I have always been this way.

Is it possible that your GF isn't playing?

When she speaks in baby-talk, don't acknowledge her. It's possible that this may be her way of getting attention. Don't give it to her. You could also try offering her an ultimatum. For example, tell her you refuse to engage in any "adult" activities with her (ie. sex) until she starts acting like one.

Set her the standard you want by ignoring her affectations and speaking properly.

Good Luck!

Act like a grown up yourself and talk to her tell her that it is not attractive !