How to drop SQL Server database currently in use and in Single user mode
To identify the session currently using the database you can try
SELECT request_session_id
FROM sys.dm_tran_locks
WHERE resource_database_id = DB_ID('G_MAIN_DE')
This built-in stored procedure shows all connections to a database
exec sp_who2
And this query kills a connection to the database, for example connection SPID #53
exec kill 53
The easiest thing to so is use SSMS Activity Monitor (Right-click server/Activity Monitor/Processes). Sort by Database. Kill whatever process has a hold onthe DB. You might also want to issue the kill command right before you set the DB to single-user and do the drop.