I don't want to go down on my gf
That seems to be too personal. Why do you think you need to tell me that?
I just can't stand the nasty taste.
I'm sure if you tactfully explain that to your partner, she should be fine with it.  Not wanting to engage in a sexual act that makes you uncomfortable is well within your rights in a relationship.  I hope she's understanding about it!
I just want to make her happy in the bedroom.
I'm glad that you want to make her happy, and if she really loves you, she wouldn't force you to do something you're not comfortable with doing. You still both can have good sex life without doing this one thing so I won't stress over this.