Thanks for that link, bubb2. The picture is illustrative of how you can have a three foot diameter gate valve and still allow one man to close it! It can also be noticed how the stem rises up through the wheel as it opens which allows it to be lubricated periodically. The packing in the stuffing box is replaceable and the box itself is adjustable for wear and compression of the packing to prevent leaks.
Here's a non-rising stem model approved by Lloyd's fro marine use.