The experimentally obtained ^{155}Gd Mössbauer effect results in isomer shifts for GdT_{2}Si_{2} compounds (where T are transition metals for the 3d, 4d and 5d series) are analysed in terms of charge-transfer effects and s, d redistribution by means of the extended Miedema and van der Woude model. The comparison between the theoretically predicted and measured values is discussed. Although these theoretical predictions of isomer shifts are in reasonable agreement with those found in the experiment, nevertheless they do not follow the experimental dependence on T metal acquired for each nd-series.