In my very first blind playthrough, I killed Gandrel for his crossbow and to increase Astarion's approval. In all subsequent playthroughs, I spared Gandrel, by never speaking to him in Act 1. **Warning, Spoilers**
The reason I spare Gandrel, is because in Act 3, Astarion will meet up with the Gur camp. You'll learn that Astarion actually kidnapped the Gur children and gave them to Cazador. Gandrel is the father to at least one of those children. If you keep Gandrel alive, it's possible for him to meet up with his kids:
