Generalised phrase structure grammar (GPSG) is a framework for describing the syntax and semantics of natural languages. GPSG was initially developed in the late 1970s by Gerald Gazdar. Other contributors include Ewan Klein, Ivan Sag, and Geoffrey Pullum. Their book Generalized Phrase Structure Grammar, published in 1985, is the main monograph on GPSG, especially as it applies to English syntax.
One of the chief goals of GPSG is to show that the syntax of natural languages can be described by context-free grammars (CFGs), with some suitable conventions intended to make writing such grammars easier for syntacticians. Among these conventions are a sophisticated feature structure system and so-called "meta-rules", which are rules generating the productions of a context-free grammar. GPSG further augments syntactic descriptions with semantic annotations that can be used to compute the compositional meaning of a sentence from its syntactic derivation tree. However, it has been argued (for example by Robert Berwick) that these extensions require parsing algorithms of a higher order of computational complexity than those used for basic CFGs.
Gerald Gazdar, and many other syntacticians, have since argued that natural languages cannot in fact be adequately described by CFGs.
GPSG is in part a reaction against transformational theories of syntax. In fact, the notational extensions to context-free grammars developed in GPSG are claimed to make transformations redundant. Most of the syntactic innovations of GPSG were subsequently incorporated into head-driven phrase structure grammar.