It will be very straight forwards once you realize how easy it is to create a canonoical link in Google Merchant Center.
First we will want to enable feed rules as an option, if not already done you can view the below video on how to do this.
Go to Google Merchant Center > Cog Icon > Data Sources
Now click on your primary feed > click on the attribute rules tab.
Click on add attribute rule. Lets call it c link, now click on the c link (do not enter)
Edit source and set to link and click on ok.
Now add a modification and choose the modify option optimize url.
Select remove all parameters and click on ok.
Now click on save as draft.
Create a new rule for canonical link
And set it to c link (our custom attribute), click on ok then save as draft.
Now apply the draft and reprocess the feed.
If you can not reprocess the feed, then most likely you are using content API, in this case you can follow this guide.