The ornamental design for a gate latch spring, as shown and described.
FIG. 1 is a front perspective view of a gate latch spring showing my new design;
FIG. 2 is a front perspective view thereof, shown in an alternate position;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a side elevational view thereof;
FIG. 6 is a front elevational view thereof; and,
FIG. 7 is a rear elevational view thereof.
FIGS. 3-7 are drawn in an enlarged scale for clarity. The broken lines in FIGS. 1 and 2 illustrate environment and forms no part of the claimed design.