I had a vision that my grandpa would passed away and it came true

If he was sick or ill then it is not a vision but just our ability to predict future events based on the present and he was old so you knew that he did not have long to live. And you also mentioned that he helped raise you so it's not a surprise that you think of him often.

I wonder if our subconscious knows when things like this are going to happen or when something is wrong, because I have had weird feelings about people/animals, only to find out they passed shortly after. I also once had a horse who was very old and not doing well at all, and was having a dream that my mother called me telling me I needed to make a decision about him immediately. I woke up from that dream to a call from my mom, telling me I needed to make a decision because he had gotten worse. With that one, it wasn't out of nowhere or anything like that because he was well into his 30's (very old for a horse) and had been slowly declining for awhile, so I think that might have been just my brain forming a dream out of my thoughts, as I was constantly worried about him and dreading what I knew was coming. However, with some of the young and healthy people and animals I knew/had that I had a strange thought about them passing (or in the case of a lot of the people I hadn't seen or heard from in years, they just popped into my mind out of nowhere), it was just odd. I attribute it to the subconscious picking up on something.

I'm sorry for your loss! My heart goes out to you. I literally had a bad dream the night before my Siamese cat died three days ago.