I personally would not like to meditate in a graveyard. But to each his own. I'm not afraid of death as is just moving on. But most folks just feed into the church lies and that is what really messes them up. I'm sorry but christianity as we know it is false. There is a true christ and I do not know his name but it is not jesus. I gravitate to Buddhism.
The advice I would give you is to light a candle of your choice (one that resonates with you) & sit quietly, undisturbed by yourself be it in a room, outside, in a care, etc. & ask your great-gran to talk with you.
Explain to her how you would like to know that she is close to you, with you, etc. & if possible would she give you a way (a sign, etc.) so that you can feel confident & comforted that it's her with you (visiting you, etc.)
I do not recommend contacting spirits that you do not know, aren't related to, etc. because 1) you don't have the experience to assist them, etc. should the need arise, 2) you would be disturbing them for your own desires, wants, etc. & I don't feel that anyone (myself included) has that right.
When we connect with loved ones they usually do not mind, however trying to connect with others for no real reason except to see if we can isn't a nice thing to do (in my opinion.)
I do Mediumship Readings, but I make sure that the person I am doing the reading for has a direct connection to the deceased, otherwise I decline to assist him/her.
Best wishes on your path & your journey <3