I'm not quite following your question. Why can't you use the automatically-managed ACM certificate within AWS, and an automatically-managed Google certificate within GCP?
Even if you want to use a single Let's Encrypt certificate on both environments, you should be able to run whatever deploy hook you need within whatever client you're using to automatically place it wherever you need (both Amazon and Google are very API-driven, after all).
There are a handful of ways people do this automatically. Two that come to mind are:
You can use a DNS-01 Challenge to generate the LE cert anywhere, including an office machine. That machine can then be scripted to install the certificates on your various platforms, or the cloud.
Some servers/balancers support cloud storage of certificates. They will read/write/detect changes from cloud storage. This allows you to handle dynamic scaling without worrying about rate limits and duplicate certificates.