Close the gate to a latched position. Lift up on the latch end of the gate until the weight no longer presses down on the latch, but not enough to make the latch ineffective. Brace scrap lumber under the gate to hold it at this height.
Position the gate wheel against the face of the gate, 3 to 4 inches in from the latch end of the gate on the side the gate swings toward. Make sure the wheel is firmly on the ground. Mark the bolt holes in the integrated bracket of the gate wheel. Drill a 1/4-inch hole through the gate in each marked location.
Thread a 1/4-inch thick carriage bolt, as long as the thickness of the bracket, plus the thickness of the gate and 1/2-inch for the nut, through each hole in the bracket and through the face of the gate. Install a fender washer and nut onto each bolt. Tighten the nuts with a socket wrench.
Position a second wheel to one side of the first, on the opposite side of the gate. Drill pilot holes as for the first wheel and install in the same way. Tighten the nuts with a socket wrench.
Open and close the gate a few times to ensure proper operation. Make adjustments as needed to the height or placement of the wheels. Check the bolts every now and then for snugness. Keeping the bolts tight will prevent unnecessary wear on the face of the gate.