Why does Jonas successfully pass on the memory of a sail to Gabe in chapter 14 of The Giver?
Jonas is able to successfully pass on the memory of a sail to Gabe because Gabe is especially receptive to them. As a newchild, he has not yet been conditioned to the community's world-view, and so is completely open to new experiences. In contrast, later in Chapter 17, Jonas tries to explain the concept of war to Fiona and Asher and is met with only irritation on the part of Asher and disbelief on the part of Fiona. Neither of them are capable of understanding new concepts because they have already formed a vision of the world as taught by the community. They do not have the memories, and would be resistant to receiving them.
Gabriel as a newchild is kind of like a blank slate, but besides that, there are indications that he might inherently possess extraordinary qualities of perception, enough to perhaps qualify him to be the new Receiver of Memories one day. He has light-colored eyes like The Giver, Jonas, and Rosemary, the only other person chosen to be the Receiver in recent memory.
The first time Jonas passes a memory to Gabe, he does not do it intentionally. Wanting only to provide comfort, he pats the child's back. While he is doing this he begins to "remember the wonderful sail that The Giver had given him not long before"; the memory passes into the child, and the child is calm. Jonas is not aware of giving the memory, and has actually been forbidden from doing so. Frightened but awed by the great power he has at hand, Jonas later passes more memories to Gabe intentionally, and decides not to confess what he has done to The Giver (Chapter 14).