Many times merchants don't want to submit all the products or can't submit all the products in the merchant center due to various reasons. 
In this app, you can control the submission of products to the merchant center by using "Collection Option". Very soon, we are going to have an option within the app, that will allow you to select or de-select products which will be submitted to the Google Merchant Center. Currently, you have to use "Collection Option" only. 
Here is the process:

1. Create a collection of products in the Shopify. App Allows you to select only one collection in the app, so make sure you create a collection in such a way that it has all the products you need to sync with Google Merchant Center.
   
2. You can keep that collection Hidden in the Shopify and your products within that will still be public and accessible by app also.
   
3. Once collection is created, go back to App's Settings page, find that collection and "Resync" the app again. Please keep in the mind if you have added new products to that collection, the app will automatically pick up the products within 30 mins. If App fails to add newly added products or remove the removed products, kindly edit any one character in the collection's description in the Shopify and save it. The app will get the collection update notification and will update products too. 

Here is a quick link to install the app: https://apps.shopify.com/google-shopping-feed

For any further query, feel free to connect with us at support@simprosys.com