I have added the new tab & added the file upload field as per my requirements. I have also added some CSV checking functionality in that code. You can modify the code as per your requirement.

So here I will be showing you the controller code to import multiple products through CSV in the gift registry.

My Upload file name is ‘importdata’.

You can add the below code

// Add back tag for redirect on the edit item page while importing items through CSV
$back = false;

if ($registry->getId()) {
    $data['image'] = $registry->getImage();

    /*** CSV Import Functionality ***/
    if ( (isset($_FILES['importdata']['name'])) && ($_FILES['importdata']['name'] != '') )  {
        $back = true;
        try  {    
            $uploaderFactory = $this->uploaderFactory->create(['fileId' => 'importdata']);
            $uploaderFactory->setAllowedExtensions(['csv']);
            $uploaderFactory->setAllowRenameFiles(true);
            $uploaderFactory->setFilesDispersion(true);

            $mediaDirectory = $this->fileSystem->getDirectoryRead(DirectoryList::MEDIA);
            $destinationPath = $mediaDirectory->getAbsolutePath('import_gift_registry_products');

            $result = $uploaderFactory->save($destinationPath);

            if (!$result) {
                throw new LocalizedException
                (
                __('File cannot be saved to path: $1', $destinationPath)
                );
            }
            else {
                $csvPath = 'import_gift_registry_products'.$result['file'];

                $mediaDirectory = $this->fileSystem->getDirectoryRead(DirectoryList::MEDIA);

                $destinationfilePath = $mediaDirectory->getAbsolutePath($csvPath);


                $f_object = fopen($destinationfilePath, "r");

                $column = fgetcsv($f_object);

                if($f_object) {
                    if( ($column[0] == 'Item Sid') && ($column[1] == 'Local UPC') && ($column[2] == 'DCS') && ($column[3] == 'Vend Code') && ($column[4] == 'Desc1') && ($column[5] == 'Attr') && ($column[6] == 'Size') && ($column[7] == 'Price') && ($column[8] == 'Qty') && ($column[9] == 'Qty Sent') && ($column[10] == 'Qty Due') && ($column[11] == 'Ext Ord Price') && ($column[12] == 'Disc %') && ($column[13] == 'Disc$') && ($column[14] == 'Disc Rsn') && ($column[15] == 'Tax%') && ($column[16] == 'Lookup')  && ($column[17] == 'Tax$')  && ($column[18] == 'Tax') && ($column[19] == 'Assoc') ) { // check the column sequence if it is not matched then import will no proceed further   

                        // Delete all items before importing
                        $items = $this->itemCollectionFactory->create()
                        ->addFieldToFilter('registry.registry_id', $registry->getId())
                        ->setOrder('item_id', 'desc');
                        $remainingItems = array();
                        foreach ($items as $deleteItem) {
                            $item = $this->itemFactory->create()->load($deleteItem->getId());
                            if($item->getQtyOrdered() == 0 && $item->getQtyReceived() == 0) {
                                $item->delete();
                            }
                            else {
                                $remainingItems[$item->getProductId()] = $item->getId();
                            }
                        }



                        $positiveCount = 0;
                        $negativeCount = 0;
                        $flag = false;
                        $notExistString = '';
                        while (($columns = fgetcsv($f_object)) !== FALSE) {

                            $sku = $columns[0];
                            $product = $this->product->getIdBySku($sku); // Get Product By SKU

                            if($product) { // Checking product is empty or not

                                if(array_key_exists($product,$remainingItems)) { //check product is exist or not in the remainingItems as a key

                                    // Update the exisitng item
                                    $item = $this->itemFactory->create()->load($remainingItems[$product]);
                                    $itemData = array('key' => $data['key'], 'form_key' => $data['form_key'], 'qty' => $columns[8], 'qty_ordered' => $item->getQtyOrdered(), 'qty_received' => $item->getQtyReceived(), 'priority_id' => $item->getPriorityId(),'note' => $item->getNote(), 'registry_id' => $registry->getId(), 'product_id' => $product, 'store_id' => 1);
                                    $item->addData($itemData);
                                    $item->save();

                                    $positiveCount++;
                                }
                                else { // Add the item

                                    $itemData = array('key' => $data['key'], 'form_key' => $data['form_key'], 'qty' => $columns[8], 'qty_ordered' => '0', 'qty_received' => '0', 'priority_id' => '','note' => '', 'registry_id' => $registry->getId(), 'product_id' => $product);


                                    $item = $this->itemFactory->create();
                                    $item->addData($itemData);
                                    $item->save();

                                    // Prepare buy request
                                    $buyRequest = new MagentoFrameworkDataObject($itemData);
                                    $buyRequest->setProduct(str_replace('product/', '', $item->getProductId()))
                                        ->setProductId($buyRequest->getProduct());

                                    // Add item in buyrequet
                                    $item->updateItem($buyRequest);
                                    $positiveCount++;
                                }
                                
                            }
                            else {
                                $flag = true;
                                $negativeCount++;
                                $notExistString .= $sku .' product not exist in the magento.<br/>';
                            }
                        }                                    

                        if($flag) {
                            $this->messageManager->addError($notExistString);
                            $this->messageManager->addError(__('A total of %1 record(s) have not added.', $negativeCount));
                            
                        }

                        $this->messageManager->addSuccessMessage(__('A total of %1 record(s) have been added.', $positiveCount));
                        
                    }
                    else {
                        $this->messageManager->addError(__("invalid Formated File"));
                    }
                }
                else {
                    $this->messageManager->addError(__("File hase been empty"));
                }

            }
        }
        catch (Exception $e) {   
           $this->messageManager->addError(__($e->getMessage()));
           $this->_redirect('*/*/edit', ['id' => $this->getRequest()->getParam('id')]);
        }
    }

    /*** CSV Import Functionality ***/
}

in the below file
File Path: app/code/Mirasvit/Giftr/Controller/Adminhtml/Registry/Save.php

Here I have added the code as per my requirement. You can change or modify the code as per your requirement.