Hi, Naylor!
Patrick, a Shopify Guru here :)
With the Goggle Merchant app, you should be able to click on “Publish to Goggle Merchant Centre” and your products will automatically be published to the centre.
You can also select individual products to upload by highlighting them and then selecting “Publish” under the Bulk action dropdown.
The update can sometimes take a while to go from Shopify to your Google merchant account. If you're still having trouble re-publishing your products feel free to reach out to us directly at support@shopify.com.
Let me know if you need anything else :).
Patrick