Created by mysqldump --gtid backup script removes any former value of
@@global.gtid_slave.pos because the script would execute

   set @@global.gtid_slave_pos = "gtid-state-of-the-donor-server"

and obviously discard any former value.
Such behavior is historical and reflects the original intention to provide support for the whole
state backup.

Thus an automatic recomputation of the slave gtid state for the partial backup has not been provided.
It can be done with improving on mysqldump to make it compose a new value as an union of the being backed up state
and @@global.gtid_slave_pos of the target server.