A Monte Carlo 3D off lattice percolation in a non-uniform reaction field is reported. Monomers are simulated as point particles of functionality (coordination number), f=3. The non-uniform field is expressed as an exponentially decaying reaction probability in one dimension tailored to simulate the photopolymerization process. It is argued that tubular reactors and other polymerizations in non-uniform fields will show similar qualitative features. Two cases of molecular mobility are studied. In the first limit molecules can only react in immediate neighborhoods corresponding to diffusion limited growth. In the second any two units can react. Results based on the distinction of whether one or both units must be activated before reaction are reported. It is found that for the particular type of non-uniform field considered reaction gradients results in inhomogeneous gelation at low conversions. Reaction gradients cause large spatial heterogeneities in the size distribution of the molecules and also introduce large polydispersities. These results are compared with recent experimental observations on photopolymerizations and tubular reactors.