What is the relationship like between your daughter and g/f?
Is it strained and they rarely speak?
Is there outward animosity between them?
I used to date a guy that had an 8 year old daughter.
The daughter was jealous of me and treated me like I was invisible.
And if her father and I were hugging, the girl would physically split us apart, and stand guard in front of her father with her arms crossed.
Needless to say, she and I didn't have a good relationship.
So my relationship with her father ended in less than a year.
If your situation is similar, then you need to talk with your daughter, and then a private conversation with the g/f.
And if it can't be worked through, then your g/f should stay away the days you have your daughter.