This sounds tough, I’m guessing you feel like your grandmother may try to prevent you being able to contact your father if she found out? It’s really your choice whether or not to try having a relationship with him, as you are an adult (as you have chosen to, I sincerely hope it goes well for you!). I can understand why she might not want you to, but I don’t agree that you shouldn’t be allowed to.
I also think it’s better to aim for a healthy relationship with him rather than strive to feel familial love for him, however. I only say this because I know you can’t force yourself to love anyone, and it’s horrible thinking you “should” love someone but realise that you don’t. Hopefully you will grow to love each other as father and daughter should, over time, though.