The state of the Group Replication related channels and threads can be observed using the regular replication_applier_status table as well. If there are many different worker threads applying transactions, then the worker tables can also be used to monitor what each worker thread is doing.
Table 17.4 replication_applier_status
The name of the Group Replication channel.
Shows whether the applier service is ON or OFF.
Shows whether there is some applier delay configured.
The number of retries performed while applying a transaction.
Transactions in this GTID set have been received by this member of the group.