Freddie here from the Shopify Support team, happy to help.
As suggested by a few others here so far, for this kind of set up your best bet would be to use an app to give you this feature. Here are a few that I found that look to be rated well by other merchants that could work for you:
These are just a few of the results I pulled up from searching in our Apps store, and you can have a search through all of the options to decide what would work best for you and what you'd want to try out, right here.
Otherwise, you can reach out to a Shopify expert to have them build you a custom solution for this into your store.
Let me know if this helps!
Shopify Support - email@example.com
if you need to offer free gifts base on cart total value, you can try to use this app:
You have 7 days trial there and app works on all devices.