The process in which a relatively unspecialized cell acquires the specialized structural and/or functional features of a garland nephrocyte. A garland nephrocyte is an insect renal cell that filters hemolymph and forms a ring with other garland nephrocytes around the esophagus. Differentiation includes the processes involved in commitment of a cell to a specific fate and its subsequent development to the mature state. [ GOC:dph http://www.ncbi.nlm.nih.gov/pubmed/19783135 GOC:mtg_kidney_jan10 http://purl.obolibrary.org/obo/CL_0000486 GOC:sart ]