Google optimize set up custom goals

[Shinya_Koizumi]

I am setting up the google optimize a/b testing but not sure how can I set up the custom goal which is customer checking out. I want to see which free gift sells more. I have set up three variants which will select one of three free gift in the shopify side and automatically added to the cart if you go over $100. I used optimize callback to decide which free gift will be chosen but not sure how can I set up the custom objects so I know customers checkout with the gift. How can I achieve this?