G2P refers to grapheme-to-phoneme conversion.  This is the process of using rules to generate a pronunciation for a word.  The rules are usually created by a automated statistical analysis of a pronunciation dictionary. 

The G2P algorithm is used to generate the most probable phone list for a word not contained in the pronunciation dictionary (i.e. out-of-vocabulary words) used to create the G2P rules.

From SPEECH and LANGUAGE PROCESSING By  Daniel Jurafsky:

The process of converting a sequence of letters into a sequence of phones is called grapheme-to-phoneme conversion, sometimes shortened g2p. The job of a grapheme-to-phoneme algorithm is thus to convert a letter string like cake into a phone string like [K EY K].