Google Analytics Enhanced Ecommerce extension is compatible with Magento Checkout.
If a Custom Checkout (OPC or other extension) is using the Magento 2 API calls the data should be pushed correctly.
If the custom checkout customized the way API calls are handled, the extensions will need compatibility adjustments with the new checkout flow. In most cases only he checkout steps are affected, the rest of the extension functionality as well as the success page purchase should work ok.
In conclusion, each custom checkout solution needs to be tested to be certain 100%.
We suggest adding a feature request for your OPC solution in Community Forum at the link below, we'll review the most used one(s) and consider an out of the box compatibility for a future release.