"Gallery Plus" is a listing upgrade. It's free in some categories like collectibles and art, but it's a paid feature in most categories. It's not free in every category just because you have a store (link).
If you used "sell similar" from a listing where it was a free feature then changed the category, that's a common way sellers get hit with the charge unexpectedly. It's also possible you checked the box by mistake. The box is right under the photo area on the listing form. The form will show you any fees before publishing, so it's important to review that area before going live - it's so easy to get into a repetitive mode and just click publish without looking at the fee amount.
Find the affected listing and uncheck the box, otherwise the Gallery Plus fee will be charged each time the listing renews @yesterdayoncemore.
