Only the sku and name parameters for product items whose category_gear attribute includes the value 86 are returned by the following query.
GET <host>/rest/<store_code>/V1/products/?searchCriteria[filter_groups][0][filters][0][field]=category_gear
&searchCriteria[filter_groups][0][filters][0][value]=86&searchCriteria[filter_groups][0][filters][0][condition_type]=finset
&fields=items[sku,name]