It's not unusual for a weep to develop from the shaft. Assuming it's
just the seal/packing, you can tighten down the gland nut a touch and
it will cease leaking for up to a year/decade. Next stage is to repack
or re-seal it. If the shaft is corroded, bin it.
Oh, they're horrible things. I had a couple of unused ones lying
around which I had to press into service as a stopgap measure, with
the intention of replacing them with proper lever valves. Guess how
long they'll be there...
Depends on the quality. There are several classes of gate valve too.
Many are made of alloys unsuitable to the water they work in. Cheap
Don't imagine a ball valve for example will last any longer if it's
made out of the same shit which they usually are. (Or worse)
A good one is made out of bronze not brass. Or stainless steel.
You won't find bronze valves at B&Q.
Most of the so-called heating engineers we have these days don't know
Taper wedge gate valves should not be installed where a dead tight
shut off is needed. Due to their design, they will inherently pass.
In practice, there are few situations needing a dead tight shut off.
But at least they can be serviced which a ball valve can't be.