Hello Sukie -
Gallery Plus and the Optional eBay Gallery images are different possible features on eBay listings.
The current functionality for listings created in Auctiva is for the Optional eBay Gallery images to be used by default, which has no additional cost on eBay US. Using Gallery Plus, is a wholly different feature that, as you have noted, does incur an additional fee on eBay.
If you have found that you have listings which make use of Gallery Plus (which can be checked under the Marketing section of the listing creation page) where you did not select such, please let us know some examples in a support case so we can look into it further.