Why were Gabe's eyes of such interest to Jonas?

The main reason why Jonas is chosen to become the new Receiver of Memory is because he has what the Elders refer to as the "Capacity to See Beyond." This means that Jonas has the ability to sense things beyond the community, into memories of the past. For the Giver, the capacity started as hearing music, but for Jonas it started as being able to see colors.
Gabriel is a baby who is going to be "released," but Jonas's father asks the committee for the chance to nurture the baby more. While Jonas's family is taking care of Gabriel, Jonas notices that the baby's eyes are special: they are pale like his own eyes and the Giver's. More than that, they seem to show a level of depth and intelligence that other people in the community don't have. Jonas learns that the pale color is called blue and later discovers that he can transmit memories to Gabriel.
The connection of blue eyes shows that the Giver, Jonas, and Gabriel may all be related or that blue eyes are an indicator of a unique trait that connects them and allows them to give and receive memories.