The ornamental design for a gaterailsupportblock, as shown and described.
FIG. 1 is a perspective view of a gate rail support block showing my new design;
FIG. 2 is a front view thereof, the rear view being the same;
FIG. 3 is a side view thereof, the opposite side view being the same; and,
FIG. 4 is a top view thereof, the bottom view being the same.
The broken lines shown in FIGS. 1 and 2 illustrate portions of the gate rail support block that form no part of the claimed design. The dash dot line shown in FIG. 1 illustrates a boundary and forms no part of the claimed design.