Stuck gate valve, any idea?

[fltcpt]

The supply main into the my house is a grayish (iron??) pipe with a small gate valve on it.
The house isn't old (built 1999) but I never turn the valve and now it is stuck (won't turn).
I don't know the reason but I speculate it is due to deposit (very hard water here).

I am not a handy person but I can wield a tool... does anybody think I can fix it
myself or do I really need a plumber. I tried pouring vinegar around it to no avail.
I would try a wrench but I worry I will crack something (the pipe, not my bones)
and send me calling for help (I won't be able to bear the embarrassment will I?)

Any other ideas how to unstuck it? The valve looks something like this,


the "knob" is about the size of my palm. Any pointer helps, thanks alot!

[HayZee518]

I would suspect that the threads inside the valve bonnet are corroded to the point, that a direct replacement is necessary. Get hold of your water department to shut off the street cock and then replace the valve. Might I suggest a new ball valve opposed to a new gate valve.
That gray pipe might be a galvanized pipe. You may need a plumber because chances are that the threads are also gunked up with corrosion.

[akaothermick]

Try backing off the packing nut, plumbers tend to give them a couple of turns when wrapping up a job, so they wont get called back if it weeps a little bit. Then try turning it.