The most recent service composition approaches rely on the mechanism, which involves scalable and decentralized execution of services. Although some formal tools have been used to this effect, they are influenced by the standard of web service orchestration and choreography based mainly on workflow languages or notation. In this paper, we describe the formal semantics of a novel service composition language through which the services are declaratively composed and executed following a peer-to-peer paradigm. The proposed language named GSLang is inspired by the GAG (Guarded Attribute Grammars) model that has been defined for the modeling collaborative systems. Pi-calculus is used to define the basic elements of the language and its operational semantics. Then its properties are highlighted through a case study.