Why dont you just ask your gf to tell you honestly how she'd feel about you pursuing something with her gf? Then see if the gf is actually interested. As for how to introduce her, use her name, or "my friend, ___", or "[your girlfriend's name]'s girlfriend, ___."
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.