In this paper, we propose a sampling-based graph-of-graphs (GoG) learning framework, SamGoG, to address two important imbalance problems in graph classification tasks: class imbalance and graph size imbalance. SamGoG constructs multiple GoGs and trains them sequentially via an efficient importance-based sampling mechanism. This sampling mechanism integrates learnable pairwise similarity and adaptive GoG node degree to enhance edge homogeneity and thus improve the quality of downstream models. SamGoG can be seamlessly integrated with various downstream GNNs, enabling efficient adaptation to graph classification tasks. Extensive experiments on benchmark datasets demonstrate that SamGoG achieves state-of-the-art performance, achieving up to 15.66% accuracy improvement and 6.7x learning speedup.