I claim the ornamental design for a gate insert, as shown and as described.
FIG. 1 is a perspective view of the gate insert with gate and an additional insert.
FIG. 2 is the front view of the gate insert with gate and a different insert.
FIG. 3 is a perspective view of the gate insert.
FIG. 4 is the front view of the gate insert
FIG. 5 is a back view of the gate insert.
FIG. 6 is a side view of the gate insert.
FIG. 7 is the other side view of the gate insert.
FIG. 8 is a top view of the gate insert; and,
FIG. 9 is bottom view of the gate insert.
The gate, wood inserts, handle, and additional gate inserts are shown in broken lines in FIGS. 1 and 2 are for illustrative purposes only to show the environment and do not form part of the claimed design.