Not enough input 
Need more info to understand the problem.
~Haiden~
You will need to elaborate more on your question as you don’t state how or where you intend to use or or what you are trying to actually filter in a gallery.
Old forum posts can help here, do a search for more…
https://www.wix.com/corvid/forum/community-discussion/filtered-gallery-display
https://www.wix.com/corvid/forum/community-discussion/filter-gallery-items
https://www.wix.com/corvid/forum/community-discussion/huge-gallery-need-some-sorting-and-filtering-help
Along with API Reference and code examples here.
https://www.wix.com/corvid/reference/wix-data.WixDataFilter.html
https://www.wix.com/corvid/reference/wix-dataset.Dataset.html#setFilter