I'm not into my gfs sexual fantasies even though I used to be
Have you told her about this?
I have not told her yet.
Well tastes change over time. It is okay to change your mind and not be into the same thing that you used to be.
She won't understand and she thinks it's impossible for me to not be interested in her fantasy.
You do not know if she will understand unless you tell her about it. Communication is important in a relationship. Talk to her about your change in taste.