I too am sorry that you are having to go through this with your grandma. Her Drs should prescribe medications that she will need, but I would also suggest looking into hospice. They are there to make their patients comfortable all the way to the end. And you and the rest of the family may be with her as much as you want.
Chris
I'm so sorry you and your Grandmother are going through this.
Depending on how aggressive the cancer is and how quickly it spreads will determine how quickly your Grandmother will start to experience discomfort and pain. Unfortunately there will be pain but I'm sure her Oncologist will arrange for her to see a pain management team to prescribe medication to help with this. If she has a build up of ascities which can be most uncomfortable, again her Onco can arrange for it to be drained.
I'm sure her Doctors will prescribe what ever medications she may need to keep her as comfortable as possible. You may like to see if there is a consultant you can speak to about palative care.
The only other thing I can suggest is to spend as much time with her as possble and perhaps do the things she enjoys while she still can. I'm really sorry, I don't know what else to suggest. Perhaps some of the ladies that have been caregivers will come on and see your post and share with you their experiences and offer more advice than I have been able to do.
Wishing you all the very best in what can only be a very difficult time for you. And please come back and post anytime you feel the need. My thoughts and prays are with you and your Grandmother... Janet