Well, perhaps the OC is a musician who plays at a local café where Gaara often goes to read. Their music catches Gaara's attention, and they start to have conversations about life and dreams. Or the OC could be a volunteer at an animal shelter and Gaara comes in looking to adopt. They bond over their love for animals and that could be the start of their relationship.
A possible plot idea is that the OC is a foreign exchange student living with Gaara's family. At first, they clash because of their different cultures and personalities. But as time passes, they learn from each other and realize they have feelings for each other. For example, the OC could introduce Gaara to new foods and music from their home country, while Gaara shows the OC the beauty of his hometown.