They're both elderly now and not in the best of health, so I'm just going to apologise to them and explain what happened and then leave it at that.
This is a decision only you can make. You could wait until your grandparents have moved on, so they are no longer troubled by your interest. On the other hand, by then your gt-uncle may also have moved on, and everything you might have learnt from him will have been lost.
Perhaps the biggest drawback with family history is that many people don't take it up until too late. You are still young and in a position to capture much of it. It would certainly be valuable to hear the story of any rift from the other side.
My wife lost touch with two cousins when they were all aged 10 to 12, because her father fell out with his sister (their mother). Fifty years later, they found each other again and get along fine. The reasons have become clearer and were basically just down to personal stubbornness, nothing to be seriously ashamed about. Your story may be similar - who knows until you find out?