Graph aggregation is an important operation when studying graphs and has been applied in many fields. The heterogeneity, fine-granularity and semantic richness of RDF graphs introduce unique requirements when aggregating the data. In this work, we propose Gagg, an RDF graph aggregation operator that is both expressive and flexible. We provide a formal definition of Gagg on top of SPARQL Algebra, define its operational semantics and describe an algorithm to answer graph aggregation queries. Our evaluation results show significant improvements in performance compared to plain-SPARQL graph aggregation.