Most PVC gate valves have an O-ring for a packing seal. I cannot see yours clearly, but if you unscrew the top bonnet, you can remove the whole wedge and steam. Take the O-ring out in the stem seal area and apply some silicon grease to the O-ring and then re-assemble. There is often some type of packing compresion nut on top but I don't see one on yours. That has worked for me on Spears gate valves that leaked at the stem. If you don't feel up to that you can also just replace the gate valve.