Critical dynamics in gelling polymeric systems is intensively investigated and debated as the experimental data interpretation and the theoretical description of the gelation phenomena are quite controversial. In order to study the viscoelastic properties and the relaxation mechanisms at the gelation transition we have recently introduced a percolation dynamic model and we here present the results of extended numerical simulations on the cubic lattice performed on parallel system CRAY-T3E. The critical behaviour of the viscosity well agrees with the prediction based on the Rouse model for polymer dynamics and with some experimental results. The elastic response in the gel phase is purely entropic and the obtained critical behaviour can be understood in terms of an entropic elasticity model for the percolating cluster. In the gelation critical regime the relaxation process shows a long-time stretched exponential decay in the time autocorrelation functions in the sol and the occurrence of a power law behaviour in the gel phase. This complex relaxation behaviour is actually observed in many gelling systems and presents interesting analogies with the glassy dynamics.