I have already setup my domain using the Google Cloud Platform, including a Load Balancer with SSL protection. Everything works on this end.
How do I connect a Marketplace Wordpress click-to-deploy creation to this existing load balancer?
If the marketplace solution is a single VM go to the instance groups menu in GCE, select unmanged groups, create a group and add the vm to the group. Then go back to the load balancer and add a backend. It will ask you what to use as a backend: endpoint (no), bucket (no) or instance group.
Go for the instance group.
Mind that LB will work only if an attached health check will detect the vm active (usually you want to check for http on listening port)