Gate valve? How would I fix leaking from the stem when the valve us on?

[bigp]

The valve might not have been operated/used for quite a while. No leakage while the valve is off. When it is turned on, water leaks from the stem. There does not seem to be a packing nut on the valve. I tend to believe it to be a gate valve. Can somebody help confirm? If yes, how can I fix the leakage?

 

[Mr_David]

Not a gate valve.
Just replace it.
You can use a 5/8 compression valve. just cut the old on off.
You only need 2 wrenches to put the new one on.

Or turn off main water. unscrew the stem cap. probably an 0-ring on the stem.

 

[JoeWest]

Sometimes you can take a length of teflon tape, lay it out on a table and roll it up long ways like a piece of spaghetti, remove the nut and wrap the teflon tape around the stem. It has worked for me