I am so sorry for this. I can feel the sadness in your post, especially about not getting to say goodbye. I can also tell you your grandmother would not want you to feel badly about something you had no control over.
Instead, she is most certainly receptive to any and all communication you have with her, now. It is a healthy part of your grieving to "talk" to her or write it out and I am sure that she will somehow know what you most need her to know.
It will help to bring you peace.
Please accept my condolences. I miss my own grandmother...she's been gone ten years as of July 3. I was in Disneyland w/my husband and son when she died and I never really got to say goodbye, either, though I know she was aware that I needed to, she gave me that chance.
Take care,
Liz M.