I'm going to explain to my gf that pit bulls can be more aggressive toward other dogs
How does your girlfriend's dogs react to other dogs?
They are friendly with other dogs in the neighbourhood
You guys are partners so you definitely should share your thoughts about her dogs. I hope she'd take it sportingly.
What can happen if I told her that I want her to get her dogs away from other dogs?
She might disagree but I can see where you are coming from. Statistically Pitbulls are an aggressive breed of dog which can suddenly cause great harm and even death to others. Try explaining your reasoning but be open to her feedback and hopefully she'll see things your way.