cellForItemAt.swift

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

  // Working with functions categories collection view
  if collectionView == self.functionsCategoriesCollectionView {
      let cellFunctionsCategories = collectionView.dequeueReusableCell(withReuseIdentifier: "cellCat", for: indexPath) as! DevicePageFunctionsCategoriesCVCell
      cellFunctionsCategories.cellTitleContent.text = ICDatabase.objects(FunctionCategory.self)[indexPath.row].name.uppercased()
      
      cellFunctionsCategories.setGradientBorder(width: 10, colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0), UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0)])
      
      if indexPath == cellSelectionIndexPath {
          if let cellToDeselect = collectionView.cellForItem(at: self.cellSelectionIndexPath) as? DevicePageFunctionsCategoriesCVCell {
              
              cellToDeselect.cellView.addGradiant(colors: [UIColor.clear.cgColor, UIColor.clear.cgColor], angle: 0)
              
              // Get all functions
              self.filtered = GlobalVariables.currentProduct.functions.filter { _ in
                  return true
              }
              
              self.functionsCollectionView.reloadData()
          }
      }
      return cellFunctionsCategories
  }
}

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    
    // Working with functions categories collection view
    if collectionView == self.functionsCategoriesCollectionView {
        
        let cell = collectionView.cellForItem(at: indexPath) as! DevicePageFunctionsCategoriesCVCell
            cell.isHidden = false
            cell.cellView.isHidden = false
            cell.isSelected = true
        
            cell.cellView.clipsToBounds = true
            cell.cellView.layer.cornerRadius = 25
            cell.cellView.addGradiant(colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0).cgColor, UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0).cgColor], angle: 45)
        
        if cellSelectionIndexPath == indexPath {
            // it was already selected
            cellSelectionIndexPath = nil
            collectionView.deselectItem(at: indexPath, animated: true)
            cell.cellView.addGradiant(colors: [UIColor.clear.cgColor, UIColor.clear.cgColor], angle: 0)
            
            self.filtered = GlobalVariables.currentProduct.functions.filter { _ in
                return true
            }
            
            self.functionsCollectionView.reloadData()
            
        } else {
            // wasn't yet selected, so let's remember it
            cellSelectionIndexPath = indexPath
            
            // Filter with seletec category name
            let cellCategoryName = ICDatabase.objects(FunctionCategory.self)[indexPath.row]
            
            self.filtered = GlobalVariables.currentProduct.functions.filter { function in
                return function.functionCategory.contains(cellCategoryName)
            }
            
            self.functionsCollectionView.reloadData()
        }
        
    }
}

The first add a gradient color only for the cell's border and second one a gradient color for the cell's background

Put this in CellForRowAtIndex method

    // you have to set default color ( this must be your deselected color)
    cellFunctionsCategories.cellView.addGradiant(colors: [UIColor.clear.cgColor, UIColor.white.cgColor], angle: 45)

        //when cell selected add gradient to fill it
        if indexPath == cellSelectionIndexPath {
              // (this must be your selected color)
              cellFunctionsCategories.cellView.addGradiant(colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0).cgColor, UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0).cgColor], angle: 45) 
        }

I have exactly the same problem with :

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        
        // Working with functions categories collection view
        if collectionView == self.functionsCategoriesCollectionView {
            cellSelectionIndexPath = indexPath
            
            let cellFunctionsCategories = collectionView.dequeueReusableCell(withReuseIdentifier: "cellCat", for: indexPath) as! DevicePageFunctionsCategoriesCVCell
            
                cellFunctionsCategories.cellView.addGradiant(colors: [UIColor.clear.cgColor, UIColor.white.cgColor], angle: 45)
            
            //when cell selected add gradient to fill it
            if indexPath == cellSelectionIndexPath {
                // (this must be your selected color)
                cellFunctionsCategories.cellView.addGradiant(colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0).cgColor, UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0).cgColor], angle: 45)
            }
            
            collectionView.reloadData()
        }
}