So, if you have domain you want to use at WordPress.com that you have hosted at Google, you have the option to map or transfer that domain.
Whether you map or transfer your existing domain, you will need to purchase a WordPress.com plan. The personal plan ($48) is the base plan that you would at least need to purchase to use the domain with your WordPress.com site.
Regarding mapping vs transferring the domain, the difference is who you want to host your domain. If you transfer the domain to WordPress.com, we’ll manage the domain. You’ll pay for the mapping and the plan here (domain is included within the plan price for the first year).