‘Sylvia’ represents a nymph or shepherdess of Arcadia.
Pastoral or Arcadian literature was a late-medieval and early-modern genre that romanticized the simplicity of rural life. Named after Arcadia, the home of the god Pan in Greek mythology, the genre depicts an uncorrupted wilderness peopled with satyrs and nymphs or shepherds and shepherdesses. Important works were Arcadia (1480) by Jacopo Sannarazo, The Countess of Pembroke's Arcadia (1580s) by Philip Sidney, and the best known today, As You Like It (1599) by William Shakespeare. Steinbeck’s title The Pastures of Heaven alludes to this pastoral genre.
The stock characters in Arcadian romances had classical names: Lysander, Damon, Strephon, etc. for the shepherds, and Sylvia, Phyllis, Cloris, etc. for the shepherdesses and nymphs. The name ‘Sylvia’ derives from Latin ‘silva’ meaning ‘wood’ (contrasting in Steinbeck with ‘George’ which derives from Greek ‘γεωργός’ meaning ‘farmer’).
A well-known Silvia appears in the play Aminta (1573) by Torquato Tasso, which portrays her romance with the shepherd boy of the title. This was adapted into the ballet Sylvia (1876) with music by Delibes. But I doubt that Steinbeck had any particular Sylvia in mind: by writing ‘a Sylvia’ he makes it clear that ‘Sylvia’ is to be understood as a type, not an individual. So the image is generic, casting George as a shepherd in an Arcadian drama, waiting in vain for a nymph to appear from the woods and fall in love with him.