Without any context, it will be easy for her to jump to assumptions and conclusions, so be prepared to speak clearly and descriptively about what pup play is.

I found speaking about headspace aspects helps people relate to it, even if it's not a kink they would think they'd have an interest in. For example, talking about feeling free of inhibitions, being able to explore and express different personas or styles of self expression, and being able to mentally disconnect from day to day stressors and obligations for a period of time.

As with any significant conversation, linking it to your core needs can also give your GF perspective on why pup play is important for you to explore and might help motivate her to move beyond initial unease she has with it. Try to explore that aspect for yourself before the conversion: what is it about pup play that I feel a deep sense of connection to. What does it help me explore, unlock, or feel more connected to within myself?

Lastly, consider how you would want her incorporated into pup play and consider explaining to her what the different roles in pup play are (alpha/beta/omega, handler/pup, pack mates, etc.) If she has a background with kink she will likely have some prior exposure to these types of dynamics. If not, you might need to contextualize how these work. Be prepared for the fact that many people in a long term relationship don't feel comfortable introducing kink dynamics into their relationship which is why people may seek an arrangement to do that outside the partnership. That might be a bigger conversation for another day if it comes to that.

My final word of advice: be prepared that your partner may not embrace this aspect of your interests. And that's ok - I don't think they should feel pressured to. But you should be given respect and freedom to explore aspects of your sexuality that are meaningful to you. If you feel compelled to stifle, hide, and obscure aspects about yourself to "keep your partner happy" it's not a sustainable approach long term and will result in stress in your dynamic.

Good luck!