How to stop a gland nut rotating too freely
Your packing is old and stiff and will no longer seal without over tightening the gland nut.

First, try laying in a small bit of new packing material on top of the old, then tighten the gland nut to get a seal. If this doesn't work (there are various reasons that it might not, like: no space in the gland to add packing, old packing too far gone, etc.) then you will need to re-pack the valve, i.e. dig out all the old packing and replace it with new.