Keep them in their individual plastic, zip lock bags they typically come in. Store them in a dark place. Preferably one that is reasonably climate controlled (not too hot, not too humid, etc…).

That’s about the best you can do. They will last a fairly long time, and if you use them to restring your harp every year or two and then replace them you should never have a problem. Stored like this they should easily last two years in storage and two years on the harp if you are a non-professional who practices an hour or two a day.

If you are a pro who performs frequently and practices

As Sylvia does in the above post, I find storing replacement harp strings by octaves in larger ziploc bags suits my needs, and then the whole lot in one even larger transparent bag, along with a small ziploc of string anchors, and a very tiny scissors. All of this is stored inside

I thought I’d add to this question as it relates to gut string longevity.

Do you mean Vanderbilt gut?