Galera cluster into Google cloud platform
but I have difficulties to see the added
  value compared to Cloud SQL instances with replication and backup. I
  am not very familiar in scaling heavy load systems.


Galera-based clusters like Percona XtraDB Cluster support true active/active multi-master, so failover is seamless because you can actually be writing on any node at any time (tho it's recommended to write only on a single node to avoid performance issues due to lock-conflicts that can happen due to the optimistic locking  strategy used by Galera).

This type of cluster can also do distributed fully-synchronous reads so you can scale heavy read loads without problems;  One shining example is Magento, which is read-heavy and has many reads that are critical -think money- and traditionally needed to be done on master to guarantee consistency, and here they can be distributed.

Keep in mind that as with any distributed CP system, Galera-cluster will add latency to the writes as it has to verify the write is valid on the other nodes, and hence it's not designed to scale writes;  That said:  it can usually withstand same write load than a regular master, as long as you keep transactions small (in terms of amounts of rows affected) and short (in regards to time spent by active transaction holding locks).

Also make sure to read list of limitations here: https://www.percona.com/doc/percona-xtradb-cluster/LATEST/limitation.html


  What strategy should I adopt in order to migrate this architecture to
  GCP ?


To decrease downtime you can migrate by setting up the GCP cluster as an asynchronous slave of the current cluster (yes, Galera cluster nodes can act as a master and/or slave in traditional, binlog based, asynchronous replication).  To prime the slave cluster you can use XtraBackup, which can take fully lock-less backups from current cluster (make sure to use latest 2.3 XtraBackup) which you can then restore on one of the nodes and allow the other two nodes to perform Snapshot State Transfer.  Then simply designate one of the nodes to become the slave, and start replication using binlog coordinates from xtrabackup_binlog_info.

Overview of steps would be:


Enable log-bin, log-slave-updates, server_id on one of the nodes from the current cluster
Take a backup of that node using xtrabackup (make sure to desync the node before: https://www.percona.com/blog/2013/10/08/taking-backups-percona-xtradb-cluster-without-stalls-flow-control/).  
Bring up one of the nodes in GCP cluster and restore the backup into it, and bootstrap the cluster with this node. Most likely use cloud storage to feed the backup to the cluster.
Bring up the other two nodes in GCP, one at a time and allow them to perform SST
Setup one of the GCP cluster nodes as async slave to the current cluster (this node also requires log-bin, log-slave-updates, server-id); this node will feed the other nodes with updates
Failover to new cluster once the slave node is caught up


Hope that helps! :)

Full disclosure: am a member of Percona Support team.