Bulk, single grain RE-Ba-Cu-O [(RE)BCO] high temperature superconductors (HTS) could potentially be used to generate stable magnetic fields for MRI and NMR. In these applications, however, the homogeneity of the magnetic field is of critical importance. As a result, the spatial distribution of critical current density, Jc, within the bulk single grain and the effects of the magnetisation process, which are primary drivers of the uniformity of the achievable trapped magnetic field, are fundamental to assessing the performance of these technologically important materials. This paper reports the systematic measurement of the distribution of Jc-B at 77 K over a vertical cross-section of a single grain along a facet line and through the seed crystal [(110)-F] at 20 positions within a 20 mm-diameter Gd-Ba-Cu-O sample in an attempt to understand and assess the distribution of Jc along this microstructural feature. A comparison of the data within the whole vertical plane across the seed measured along the a or b direction within the [(100)-a] plane shows that Jc-B at 77 K at the facet line is more than 10% higher for applied fields between 0.2 T to 2.5 T. The effect of the Jc-B relationship of the facet line on the overall trapped field measured in an individual bulk sample was investigated by measuring the magnitudes of trapped fields and their contour maps for sections cut from four single grain samples of GdBCO-Ag at different sizes and shapes parallel to the ab-plane from the top to the bottom of the bulk sample. Based on the results reported here, we demonstrate a method to achieve more uniform trapped fields through an optimal arrangement of an assembly of sections of individual GdBCO single grains.