The Constrained Application Protocol (CoAP) is a specialized Web transfer protocol for use with constrained devices and networks in the Internet of Things (IoT). Although a single CoAP service provided by a CoAP-enabled device has its own value for its users, the functionality provided by an individual CoAP service is limited. Hence, an efficient method of manipulating a set of CoAP services concurrently and processing their output data is needed. However, existing approaches require users to manually discover services and create mashups. Manual discovery and mashups are quite time-consuming tasks especially in an IoT paradigm. In this paper, we propose a novel CoAP Service Gateway (CoAP-SG) as an intermediate network node between a resource-constrained network and the Internet. The CoAP-SG enables users to create a CoAP service mashup automatically through a single HTTP request. In addition, semantic similarity is proposed to select similar services in case there is no exact matchings. Experimental results with a prototype implementation show that the proposed approach is feasible in terms of execution time and scalability.