SET @old_gtid_slave_pos= @@gtid_slave_pos;
SET GLOBAL gtid_slave_pos= '';
SELECT @@gtid_slave_pos;
@@gtid_slave_pos

SET GLOBAL gtid_slave_pos= '1-2-3';
SELECT variable_value FROM information_schema.global_variables
WHERE variable_name='gtid_slave_pos';
variable_value
1-2-3
SET @@global.gtid_slave_pos= '1-2-4';
SELECT @@gtid_slave_pos;
@@gtid_slave_pos
1-2-4
SET GLOBAL gtid_slave_pos= ' 1-2-3';
SELECT @@gtid_slave_pos;
@@gtid_slave_pos
1-2-3
SET GLOBAL gtid_slave_pos= '1-2-3, 2-4-6';
SELECT @@gtid_slave_pos;
@@gtid_slave_pos
1-2-3,2-4-6
SET GLOBAL gtid_slave_pos= '-1-2-3';
ERROR HY000: Could not parse GTID list
SET GLOBAL gtid_slave_pos= '1-2 -3';
ERROR HY000: Could not parse GTID list
SET GLOBAL gtid_slave_pos= '1-2-3 ';
ERROR HY000: Could not parse GTID list
SET GLOBAL gtid_slave_pos= '1-2-3,2-4';
ERROR HY000: Could not parse GTID list
SET GLOBAL gtid_slave_pos= '0-1-10,0-2-20';
ERROR HY000: GTID 0-2-20 and 0-1-10 conflict (duplicate domain id 0)
SET GLOBAL gtid_slave_pos= '0-1-10,1-2-20,2-3-30,1-20-200,3-4-1';
ERROR HY000: GTID 1-20-200 and 1-2-20 conflict (duplicate domain id 1)
SET gtid_slave_pos= '';
ERROR HY000: Variable 'gtid_slave_pos' is a GLOBAL variable and should be set with SET GLOBAL
SET SESSION gtid_slave_pos= '';
ERROR HY000: Variable 'gtid_slave_pos' is a GLOBAL variable and should be set with SET GLOBAL
SET GLOBAL gtid_slave_pos= '1-2-3,2-4-6';
SELECT @@gtid_slave_pos;
@@gtid_slave_pos
1-2-3,2-4-6
SET GLOBAL gtid_slave_pos= DEFAULT;
ERROR 42000: Variable 'gtid_slave_pos' doesn't have a default value
SELECT @@session.gtid_slave_pos;
ERROR HY000: Variable 'gtid_slave_pos' is a GLOBAL variable
SET GLOBAL gtid_slave_pos= @old_gtid_slave_pos;