Нашли решение, нужно преобразовать структуру в строку
func GetAllBusinessPartners(db *gorm.DB, w http.ResponseWriter, r *http.Request) {
vars := mux.Vars(r)
guid := vars["id"]
results := []model.BusinessPartner{}
pr := []model.PartnerRelationship{}
db.Table("partner_relationships").Select("partner_id").Where("user_id = ?", guid).Find(&pr)
var s []string
for _, v := range pr {
s = append(s, v.PartnerID.String())
}
fmt.Printf("%q\n", s)
db.Table("users").Select("users.user_id, users.first_name, users.last_name, users.profile_image, business_informations.title, business_informations.business_name").
Joins("JOIN business_informations ON users.user_id = business_informations.user_id").
Where("users.user_id != ?", guid).
// Where("users.user_id NOT IN (?)", []string{"fbfda7f4-3691-44d0-89f2-8ccdaf3abe88", "12caf34b-42d0-4ac7-a347-a6ae1a88cb43"}).
Where("users.user_id NOT IN (?)", s).
Scan(&results)
respondJSON(w, http.StatusOK, results)
}