I'm in a very similar situation, but with my great grandmother who basically raised me. I'll pray for your grandfather's life and soul.
Be strong anon and cherish all the memories your have with your grandparents. Hope you feeling alright.
To God we belong and to Him we will return.
That sucks. I hope you can get through it okay. I'm not going to lie; it's almost certainly going to be very tough. My grandpa died of cancer years ago and held on a lot longer than they thought. I thought I was mentally prepared for it, but the first week after that was one of the worst of my life. I'm largely over it now (aside from the effect his death has had on my family) and can think of all the good times, but I still hope I can connect with his spirit/consciousness again someday.
Hope you had a good chat together.