When attempting to deleted the unit from GMS 9.3 Console interface we some time are unable to delete it due to internal error.
We can login to database to delete the unit from GMS
Login to identify the unit to be removed .
Tz 300 is the test device shown in the below example on GMS 9.3 console interface GUI .

Login to GMS database SGMSDB using any SQL browser Heidi SQL browser used in the example here.
Once logged in.
Run following query
select * from params.

Select the row that contains the name of the unit that you are unable to remove or delte from GMS GUI.
Right click the row and delete it .

Unit would be removed from GMS 9.3