This paper presented a simple method to improve the connection degree of GFRP-concrete composite decks and further improved the ductility of the composite decks. The connection strength of GFRP-concrete interface was changed by using different gravel coverage. The bending stiffness was significantly changed due to the connection mechanism change after the GFRP-concrete interface failed, and the ductility was also further improved. Five full-size T-shape GFRP-concrete composite decks were tested to explore the difference of the mechanical properties of composite decks with different connection degree. In the experiment results, deformation, slip at the end of decks, strain distribution and failure modes were mainly examined. The experimental results show that although the GFRP-concrete interface of the specimen with unbonded interface is smooth, partial composite action between GFRP plate and concrete can be observed. A variable gravel coverage changed the failure mode, ultimate load, connection degree and ductility. A reasonable gravel coverage can provide an effective composite action before slip at the end of decks occurs, offer a large deformation after slip occurs, and not significantly reduce the ultimate load. At last, the consistency of theoretical results with experimental results demonstrates that the cracking load and shear capacity can be predicted by using the methods in the paper.